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    作者:佳明妈 来源:web前端开发 2016-09-09 人气:
    http2协议规范描述了一种优化的超文本传输协议(HTTP)。HTTP/2通过引进报头字段压缩以及多路复用来更有效利用网络资源、减少感知延迟。另外还介绍了服务器推送规范。 本文档保持对

    《http2协议中文版》下载(word文档格式) 下载地址在文章最后




    1 http2协议介绍

    The Hypertext Transfer Protocol (HTTP) is a wildly successful protocol. However, the HTTP/1.1 message format ([RFC7230], Section 3) was designed to be implemented with the tools at hand in the 1990s, not modern Web application performance. As such it has several characteristics that have a negative overall effect on application performance today.

    超文本传输协议(HTTP)是一个非常成功的协议。 但是HTTP/1.1 是针对90年代的情况而不是现代web应用的性能而设计的,导致它的一些特点已经对现代应用程序的性能产生负面影响。

    In particular, HTTP/1.0 only allows one request to be outstanding at a time on a given connection. HTTP/1.1 pipelining only partially addressed request concurrency and suffers from head-of-line blocking. Therefore, clients that need to make many requests typically use multiple connections to a server in order to reduce latency.


    Furthermore, HTTP/1.1 header fields are often repetitive and verbose, which, in addition to generating more or larger network packets, can cause the small initial TCP [TCP] congestion window to quickly fill. This can result in excessive latency when multiple requests are made on a single new TCP connection.


    This specification addresses these issues by defining an optimized mapping of HTTP's semantics to an underlying connection. Specifically, it allows interleaving of request and response messages on the same connection and uses an efficient coding for HTTP header fields. It also allows prioritization of requests, letting more important requests complete more quickly, further improving performance.

    本协议通过定义一个优化的基础连接的HTTP语义映射来解决这些问题。 具体地,它允许在同一连接上交错地建立请求和响应消息,并使用高效率编码的HTTP报头字段。 它还允许请求的优先级,让更多的重要的请求更快速的完成,进一步提升了性能。

    The resulting protocol is designed to be more friendly to the network, because fewer TCP connections can be used in comparison to HTTP/1.x. This means less competition with other flows, and longer-lived connections, which in turn leads to better utilization of available network capacity.

    最终协议设计为对网络更友好,因为它相对HTTP/1.x减少了TCP连接。 这意味着与其他流更少的竞争以及更长时间的连接,从而更有效地利用可用的网络容量。

    Finally, this encapsulation also enables more efficient processing of messages through use of binary message framing.


    2 HTTP / 2协议概述

    HTTP/2 provides an optimized transport for HTTP semantics. HTTP/2 supports all of the core features of HTTP/1.1, but aims to be more efficient in several ways.

    HTTP/2 提供了HTTP语义的传输优化。HTTP/2支持所有HTTP/1.1的核心特征,并且在不同的方面做的更高效。

    The basic protocol unit in HTTP/2 is a frame (Section 4.1). Each frame type serves a different purpose. For example, HEADERS and DATA frames form the basis of HTTP requests and responses (Section 8.1); other frame types like SETTINGS, WINDOW_UPDATE, and PUSH_PROMISE are used in support of other HTTP/2 features.

    HTTP/2中基本的协议单位是帧。每个帧都有不同的类型和用途。例如,报头(HEADERS)和数据(DATA)帧组成了基本的HTTP 请求和响应;其他帧例如 设置(SETTINGS),窗口更新(WINDOW_UPDATE), 和推送承诺(PUSH_PROMISE)是用来实现HTTP/2的其他功能。

    Multiplexing of requests is achieved by having each HTTP request-response exchanged assigned to a single stream (Section 5). Streams are largely independent of each other, so a blocked or stalled request does not prevent progress on other requests.


    Flow control and prioritization ensure that it is possible to properly use multiplexed streams. Flow control (Section 5.2) helps to ensure that only data that can be used by a receiver is transmitted. Prioritization (Section 5.3) ensures that limited resources can be directed to the most important requests first.


    HTTP/2 adds a new interaction mode, whereby a server can push responses to a client (Section 8.2). Server push allows a server to speculatively send a client data that the server anticipates the client will need, trading off some network usage against a potential latency gain. The server does this by synthesizing a request, which it sends as a PUSH_PROMISE frame. The server is then able to send a response to the synthetic request on a separate stream.


    Frames that contain HTTP header fields are compressed (Section 4.3). HTTP requests can be highly redundant, so compression can reduce the size of requests and responses significantly.



    The HTTP/2 specification is split into four parts:


    • Starting HTTP/2 (Section 3) covers how an HTTP/2 connection is initiated.
    • The framing (Section 4) and streams (Section 5) layers describe the way HTTP/2 frames are structured and formed into multiplexed streams.
    • Frame (Section 6) and error (Section 7) definitions include details of the frame and error types used in HTTP/2.
    • HTTP mappings (Section 8) and additional requirements (Section 9) describe how HTTP semantics are expressed using frames and streams.

    • 启动HTTP/2(章节3)包含了一个HTTP/2连接是如何初始化的。

    • 帧(章节4)和流层(章节5)描述了 HTTP/2流的结构以及如何形成复用流的。
    • 帧(章节6)和错误码(章节7)定义了HTTP/2中使用的流和错误类型的详细内容。
    • HTTP寻址(章节8)和拓展需求(章节9)描述了HTTP语义化是如何由帧和流表达的。

    While some of the frame and stream layer concepts are isolated from HTTP, the intent is not to define a completely generic framing layer. The framing and streams layers are tailored to the needs of the HTTP protocol and server push.



    The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119 [RFC2119].

    文档中出现的关键字“必须”,“绝对不能”,“要求”,“应”,“不应”,“应该”,“不应该”,“建议”,“可以”及“可选”可通过在 RFC 2119 的解释进行理解【RFC2119】

    All numeric values are in network byte order. Values are unsigned unless otherwise indicated. Literal values are provided in decimal or hexadecimal as appropriate. Hexadecimal literals are prefixed with 0x to distinguish them from decimal literals.


    The following terms are used:

    • client: The endpoint initiating the HTTP/2 connection. connection:A transport-level connection between two endpoints.
    • connection error: An error that affects the entire HTTP/2 connection.
    • endpoint: Either the client or server of the connection.
    • frame: The smallest unit of communication within an HTTP/2 connection, consisting of a header and a variable-length sequence of bytes structured according to the frame type.
    • peer: An endpoint. When discussing a particular endpoint, "peer" refers to the endpoint that is remote to the primary subject of discussion.
    • receiver: An endpoint that is receiving frames.
    • sender: An endpoint that is transmitting frames.
    • server: The endpoint which did not initiate the HTTP/2 connection.
    • stream: A bi-directional flow of frames across a virtual channel within the HTTP/2 connection.
    • stream error: An error on the individual HTTP/2 stream.


    • 客户端:发起HTTP/2请求的端点
    • 连接:在两个端点之间的传输层级别的连接
    • 连接错误:整个HTTP/2连接过程中发生的错误
    • 端点:连接的客户端或服务器
    • 帧:HTTP/2.0通信连接中的最小单元,包括根据帧类型结构的字节的报头和可变长度 的序列
    • 对等端:一个端点。当讨论特定的端点时,“对等端”指的是讨论的主题的远程端点
    • 接收端:正在接收帧的端点
    • 发送端:正在传输帧的端点
    • 服务端:不是启动HTTP/2连接的端点
    • 流:一个双向字节帧流穿过HTTP/2连接中的虚拟通道
    • 流错误:一个HTTP/2流中的错误

    3 启动HTTP/2

    An HTTP/2 connection is an application level protocol running on top of a TCP connection ([TCP]). The client is the TCP connection initiator.


    HTTP/2 uses the same "http" and "https" URI schemes used by HTTP/1.1. HTTP/2 shares the same default port numbers: 80 for "http" URIs and 443 for "https" URIs. As a result, implementations processing requests for target resource URIs likehttp://example.org/foo or https://example.com/bar are required to first discover whether the upstream server (the immediate peer to which the client wishes to establish a connection) supports HTTP/2.

    HTTP/2使用与HTTP/1.1相同的"http"和"https" 资源标识符(URI)。使用相同的默认端口:"http" 的80端口及“https”的443端口。因此,实现对例如http://example.org/foo或https://example.com/bar目标资源的URI请求处理需要首先确定上游服务端(当前客户端希望建立连接的对等端)是否支持HTTP/2。

    The means by which support for HTTP/2 is determined is different for "http" and "https" URIs. Discovery for "http" URIs is described in Section 3.2. Discovery for "https" URIs is described in Section 3.3.

    这意味着检测“http” 及“https” 的URIs是否支持HTTP/2的方法是不一样的。检测"http"URIs在章节3.2中描述。检测"https"URIs 在章节3.3中描述。

    3.1 HTTP/2版本定义

    The protocol defined in this document has two identifiers.

    • The string "h2" identifies the protocol where HTTP/2 uses TLS [TLS12]. This identifier is used in the TLS application layer protocol negotiation extension (ALPN) [TLSALPN] field and any place that HTTP/2 over TLS is identified.

      The "h2" string is serialized into an ALPN protocol identifier as the two octet sequence: 0x68, 0x32.

    • The string "h2c" identifies the protocol where HTTP/2 is run over cleartext TCP. This identifier is used in the HTTP/1.1 Upgrade header field and any place that HTTP/2 over TCP is identified.


    • 字符"h2"表示HTTP/2协议使用TLS[TLS]。这种方式用在HTTP/1.1的升级字段、TLS 应用层协议协商扩展字段以及其他需要定义协议的地方。当在定义ALPN协议(序列化的字节)中序列化时。 "h2"字符序列化到 ALPN 协议中变成两个字节序列:0x68,0x32。

    • 字符"h2c" 表示HTTP/2协议运行在明文TCP上。这个标识用在HTTP/1.1 升级报头字段以及任何TCP是确定的地方。

    Negotiating "h2" or "h2c" implies the use of the transport, security, framing and message semantics described in this document.

    用到"h2" 或者 "h2c" 表明使用文档中定义的传输、安全、帧及语义化消息。

    Only implementations of the final, published RFC can identify themselves as "h2" or "h2c". Until such an RFC exists, implementations MUST NOT identify themselves using these strings.


    Examples and text throughout the rest of this document use "h2" as a matter of editorial convenience only. Implementations of draft versions MUST NOT identify using this string.


    Implementations of draft versions of the protocol MUST add the string "-" and the corresponding draft number to the identifier. For example, draft-ietf-httpbis-http2-11 over TLS is identified using the string "h2-11".

    依据草案版本实现的协议必须添加字符"-"及相对应的草案版本进行标识。例如,基于TLS 的草案draft-ietf-httpbis-http2-11 需要使用字符"h2-11"进行标识。

    Non-compatible experiments that are based on these draft versions MUST append the string "-" and an experiment name to the identifier. For example, an experimental implementation of packet mood-based encoding based on draft-ietf-httpbis-http2-09 might identify itself as "h2-09-emo". Note that any label MUST conform to the "token" syntax defined in Section 3.2.6 of [RFC7230]. Experimenters are encouraged to coordinate their experiments on the ietf-http-wg@w3.org mailing list.

    基于这些草案版本的不兼容的实验必须在标识符中添加字符"-"及实验名称。例如,基于draft-ietf-httpbis-http2-09草案的情绪编码实验实现必须使用类似"h2-09-emo"的标识符。需要注意的是任何标签必须符合[RFC7230]章节3.2.6 定义的"token"语法。鼓励实验者提交实验到ietf-http-wg@w3.org 的邮件列表中。

    3.2 Starting HTTP/2 for "http" URIs 针对"http"启动HTTP/2

    A client that makes a request to an "http" URI without prior knowledge about support for HTTP/2 uses the HTTP Upgrade mechanism (Section 6.7 of [RFC7230]). The client makes an HTTP/1.1 request that includes an Upgrade header field identifying HTTP/2 with the "h2c" token. The HTTP/1.1 request MUST include exactly one HTTP2-Settings (Section 3.2.1) header field.

    客户端无法预知服务端是否支持HTTP/2.0 的情况下使用HTTP升级机制发起“http” URI请求([RFC7230] 章节6.7)。客户端发起一个http1.1请求,其中包含识别HTTP/2的升级报头字段与h2c token。HTTP/1.1必须包含一个确切的HTTP2-Settings中的报头字段。


    GET /default.htm HTTP/1.1
    Host: server.example.com
    Connection: Upgrade, HTTP2-Settings
    Upgrade: h2c
    HTTP2-Settings: <base64url encoding of HTTP/2 SETTINGS payload>

    Requests that contain an entity body MUST be sent in their entirety before the client can send HTTP/2 frames. This means that a large request entity can block the use of the connection until it is completely sent.


    If concurrency of an initial request with subsequent requests is important, a small request can be used to perform the upgrade to HTTP/2, at the cost of an additional round-trip.


    A server that does not support HTTP/2 can respond to the request as though the Upgrade header field were absent:


    HTTP/1.1 200 OK
    Content-Length: 243
    Content-Type: text/html

    A server MUST ignore a "h2" token in an Upgrade header field. Presence of a token with "h2" implies HTTP/2 over TLS, which is instead negotiated as described in Section 3.3.

    服务端必须忽略升级报头字段中的“h2” token。“h2” token基于TLS实现的HTTP/2,协商方法在章节3.3中定义。

    A server that supports HTTP/2 can accept the upgrade with a 101 (Switching Protocols) response. After the empty line that terminates the 101 response, the server can begin sending HTTP/2 frames. These frames MUST include a response to the request that initiated the Upgrade.


    HTTP/1.1 101 Switching Protocols
    Connection: Upgrade
    Upgrade: h2c
    [ HTTP/2 connection ...

    The first HTTP/2 frame sent by the server is a SETTINGS frame (Section 6.5). Upon receiving the 101 response, the client sends a connection preface (Section 3.5), which includes a SETTINGS frame.


    The HTTP/1.1 request that is sent prior to upgrade is assigned stream identifier 1 and is assigned default priority values (Section 5.3.5). Stream 1 is implicitly half closed from the client toward the server, since the request is completed as an HTTP/1.1 request. After commencing the HTTP/2 connection, stream 1 is used for the response.

    (*** 升级前所发送的HTTP/1.1请求被派送到标示流1并将赋予最高优先级。) HTTP/1.1最开始用来升级到2.0的请求用1来标示流并将赋予最高优先级。1流对发送到服务端的客户端是隐式半封闭的,因为这个请求已经作为HTTP/1.1请求完成了。HTTP/2连接开始后, 1流在响应中使用。

    3.2.1 HTTP2-Settings Header Field HTTP2-Setting报头字段

    A request that upgrades from HTTP/1.1 to HTTP/2 MUST include exactly one HTTP2-Settings header field. The HTTP2-Settings header field is a hop-by-hop header field that includes parameters that govern the HTTP/2 connection, provided in anticipation of the server accepting the request to upgrade.

    从HTTP/1.1升级到HTTP/2的请求必须包含一个确切的HTTP2-Settings报头字段。HTTP2-Settings 的报头字段是逐跳报头字段,它包含管理HTTP/2连接参数。这是从对于服务端接受升级请求的预测中所获取的。

    HTTP2-Settings    = token68

    A server MUST reject an attempt to upgrade if this header field is not present. A server MUST NOT send this header field.


    The content of the HTTP2-Settings header field is the payload of a SETTINGS frame (Section 6.5), encoded as a base64url string (that is, the URL- and filename-safe Base64 encoding described in Section 5 of [RFC4648], with any trailing '=' characters omitted). The ABNF [RFC5234] production for token68 is defined in Section 2.1 of [RFC7235].

    HTTP2-Settings报头字段的内容是设置(SETTINGS)帧的有效载体,使用base64url字符编码(URL及文件名安全的Base64编码,编码描述在[RFC4648] 章节5中,忽略任何“=”字符。) ABNF[RFC5234]产品中对token68的定义在[RFC7235] 章节2.1中。

    As a hop-by-hop header field, the Connection header field MUST include a value of HTTP2-Settings in addition to Upgrade when upgrading to HTTP/2.


    A server decodes and interprets these values as it would any other SETTINGS frame. Acknowledgement of the SETTINGS parameters (Section 6.5.3) is not necessary, since a 101 response serves as implicit acknowledgment. Providing these values in the Upgrade request ensures that the protocol does not require default values for the above SETTINGS parameters, and gives a client an opportunity to provide other parameters prior to receiving any frames from the server.


    3.3 Starting HTTP/2 for "https" URIs 针对“https”启动HTTP/2

    A client that makes a request to an "https" URI without prior knowledge about support for HTTP/2 uses TLS [TLS12] with the application layer protocol negotiation extension [TLSALPN].

    客户端在不了解服务端是否支持HTTP/2的时候,会使用TSL [TLS12] 于其应用层协议协商扩展 [TLSALPN]。

    HTTP/2 over TLS uses the "h2" application token. The "h2c" token MUST NOT be sent by a client or selected by a server.

    使用TLS的HTTP/2 使用"h2"程序token。“h2c”token绝对不能由客户端或者选定的服务端发送。

    Once TLS negotiation is complete, both the client and the server send a connection preface (Section 3.5).


    3.4 Starting HTTP/2 with Prior Knowledge 先验下启动HTTP/2

    A client can learn that a particular server supports HTTP/2 by other means. For example, [ALT-SVC] describes a mechanism for advertising this capability.


    A client MAY immediately send HTTP/2 frames to a server that is known to support HTTP/2, after the connection preface (Section 3.5). A server can identify such a connection by the use of the "PRI" method in the connection preface. This only affects the establishment of HTTP/2 connections over cleartext TCP; implementations that support HTTP/2 over TLS MUST use protocol negotiation in TLS [TLSALPN].


    Prior support for HTTP/2 is not a strong signal that a given server will support HTTP/2 for future connections. It is possible for server configurations to change; for configurations to differ between instances in clustered server; or network conditions to change.


    3.5 HTTP/2 Connection Preface HTTP/2连接序言

    Upon establishment of a TCP connection and determination that HTTP/2 will be used by both peers, each endpoint MUST send a connection preface as a final confirmation and to establish the initial SETTINGS parameters for the HTTP/2 connection.


    The client connection preface starts with a sequence of 24 octets, which in hex notation are:



    (the string PRI * HTTP/2.0 SM ). This sequence is followed by a SETTINGS frame (Section 6.5). The SETTINGS frame MAY be empty. The client sends the client connection preface immediately upon receipt of a 101 Switching Protocols response (indicating a successful upgrade), or as the first application data octets of a TLS connection. If starting an HTTP/2 connection with prior knowledge of server support for the protocol, the client connection preface is sent upon connection establishment.

    (字符串PRI * HTTP/2.0 SM )。这个序列后跟着一个设置帧,其可为空帧。客户端在收到101转换协议响应(升级成功指示)后马上发送客户端连接序言,或者作为TLS连接的第一个应用数据字节。如果在预先知道服务器支持HTTP/2的情况下启动HTTP/2连接,客户端连接序言在连接建立后发送。

    The client connection preface is selected so that a large proportion of HTTP/1.1 or HTTP/1.0 servers and intermediaries do not attempt to process further frames. Note that this does not address the concerns raised in [TALKING].


    The server connection preface consists of a potentially empty SETTINGS frame (Section 6.5) that MUST be the first frame the server sends in the HTTP/2 connection.


    To avoid unnecessary latency, clients are permitted to send additional frames to the server immediately after sending the client connection preface, without waiting to receive the server connection preface. It is important to note, however, that the server connection preface SETTINGS frame might include parameters that necessarily alter how a client is expected to communicate with the server. Upon receiving the SETTINGS frame, the client is expected to honor any parameters established.


    Clients and servers MUST terminate the TCP connection if either peer does not begin with a valid connection preface. A GOAWAY frame (Section 6.8) can be omitted if it is clear that the peer is not using HTTP/2.


    4 HTTP Frames HTTP帧

    Once the HTTP/2 connection is established, endpoints can begin exchanging frames.


    4.1 Frame Format 帧格式

    All frames begin with a fixed 8-octet header followed by a payload of between 0 and 16,383 octets.


      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     | R |     Length (14)           |   Type (8)    |   Flags (8)   |
     |R|                 Stream Identifier (31)                      |
     |                   Frame Payload (0...)                      ...

    The fields of the frame header are defined as:

    • R :A reserved 2-bit field. The semantics of these bits are undefined and the bits MUST remain unset (0) when sending and MUST be ignored when receiving.
    • Length : The length of the frame payload expressed as an unsigned 14-bit integer. The 8 octets of the frame header are not included in this value.
    • Type : The 8-bit type of the frame. The frame type determines the format and semantics of the frame. Implementations MUST ignore and discard any frame that has a type that is unknown.
    • Flags :An 8-bit field reserved for frame-type specific boolean flags. Flags are assigned semantics specific to the indicated frame type. Flags that have no defined semantics for a particular frame type MUST be ignored, and MUST be left unset (0) when sending.
    • R: A reserved 1-bit field. The semantics of this bit are undefined and the bit MUST remain unset (0) when sending and MUST be ignored when receiving.
    • Stream Identifier: A 31-bit stream identifier (see Section 5.1.1). The value 0 is reserved for frames that are associated with the connection as a whole as opposed to an individual stream.


    • R : 保留的2位字段。这些字节的语义是未定义的,并且在发送的时候必须保持未设置(0),接收的时候必须被忽略此字段。
    • Length : 14位无符号整数的帧主体长度。8字节长度的帧报头信息不计算在此内。
    • Type : 帧的8位类型。帧类型定义了剩余的帧报头和帧主体将如何被解释。具体实现必须在收到未知帧类型(任何未在文档中定义的帧)时作为连接错误中的类型协议错误(PROTOCOL_ERROR)处理。
    • Flags : 为帧类型保留的8字节字段有具体的布尔标识。 标识针对确定的帧类型赋予特定的语义。确定帧类型定义语义以外的标示必须被忽略,并且必须在发送的时候保留未设置(0)。
    • R : 1位的保留字段。这个字段的语义未设置并且必须在发送的时候保持未设置(0),在接受的时候必须被忽略。
    • Stream Identifier : 31字节的流标识符(见StreamIdentifiers)。0是保留的,标明帧是与连接相关作为一个整体而不是一个单独的流。

    The structure and content of the frame payload is dependent entirely on the frame type.


    4.2 Frame Size 帧大小

    The maximum size of a frame payload varies by frame type. The absolute maximum size of a frame payload is 214-1 (16,383) octets, meaning that the maximum frame size is 16,391 octets. All implementations MUST be capable of receiving and minimally processing frames up to this maximum size.

    帧主体的最大长度限制因不同的帧类型而不同。最大帧主体的绝对长度是 $2^{14}-1$ (16,383)字节,表示最大的帧长度是16,391字节。所有的实现必须具备接收和处理此最大长度帧的能力。

    Certain frame types, such as PING (Section 6.7), impose additional limits on the amount of payload data allowed.


    If a frame size exceeds any defined limit, or is too small to contain mandatory frame data, the endpoint MUST send a FRAME_SIZE_ERROR error. A frame size error in a frame that could alter the state of the entire connection MUST be treated as a connection error (Section 5.4.1); this includes any frame carrying a header block (Section 4.3) (that is, HEADERS, PUSH_PROMISE, and CONTINUATION), SETTINGS, and any WINDOW_UPDATE frame with a stream identifier of 0.


    4.3 Header Compression and Decompression 报头压缩和解压缩

    A header field in HTTP/2 is a name with one or more associated values. They are used within HTTP request and response messages as well as server push operations (see Section 8.2).


    Header sets are collections of zero or more header fields. When transmitted over a connection, a header set is serialized into a header block using HTTP Header Compression [COMPRESSION]. The serialized header block is then divided into one or more octet sequences, called header block fragments, and transmitted within the payload of HEADERS (Section 6.2), PUSH_PROMISE (Section 6.6) or CONTINUATION (Section 6.10) frames.


    HTTP Header Compression does not preserve the relative ordering of header fields. Header fields with multiple values are encoded into a single header field using a special delimiter (see Section, this preserves the relative order of values for that header field.

    HTTP报文头压缩并不保留报头字段的相关顺序。具有多个值的报头字段使用特定的分割器被编码分割到一个单独的报头区域(见 章节8.1.2.3 HeaderOrdering),这保留了该报头字段中各种值的对应顺序。

    The Cookie header field [COOKIE] is treated specially by the HTTP mapping (see Section


    A receiving endpoint reassembles the header block by concatenating its fragments, then decompresses the block to reconstruct the header set.

    A complete header block consists of either:

    • a single HEADERS or PUSH_PROMISE frame, with the END_HEADERS flag set, or
    • a HEADERS or PUSH_PROMISE frame with the END_HEADERS flag cleared and one or more CONTINUATION frames, where the last CONTINUATION frame has the END_HEADERS flag set.



    • 一个包含报头终止标记集合的单独的报头HEADERS 或 推送承诺PUSH_PROMISE帧,或者
    • 一个报头终止标记被清除的报头HEADERS 或 推送承诺PUSH_PROMISE帧以及一个或多个延续CONTINUATION帧,最后一个延续CONTINUATION帧拥有报头终止标记设置。

    Header compression is stateful, using a single compression context for the entire connection. Each header block is processed as a discrete unit. Header blocks MUST be transmitted as a contiguous sequence of frames, with no interleaved frames of any other type or from any other stream. The last frame in a sequence of HEADERS or CONTINUATION frames MUST have the END_HEADERS flag set. The last frame in a sequence of PUSH_PROMISE or CONTINUATION frames MUST have the END_HEADERS flag set. This allows a header block to be logically equivalent to a single frame.


    Header block fragments can only be sent as the payload of HEADERS, PUSH_PROMISE or CONTINUATION frames, because these frames carry data that can modify the compression context maintained by a receiver. An endpoint receiving HEADERS, PUSH_PROMISE or CONTINUATION frames MUST reassemble header blocks and perform decompression even if the frames are to be discarded. A receiver MUST terminate the connection with a connection error (Section 5.4.1) of type COMPRESSION_ERROR if it does not decompress a header block.


    5 Streams and Multiplexing 流和多路复用

    A "stream" is an independent, bi-directional sequence of frames exchanged between the client and server within an HTTP/2 connection. Streams have several important characteristics:

    • A single HTTP/2 connection can contain multiple concurrently open streams, with either endpoint interleaving frames from multiple streams.
    • Streams can be established and used unilaterally or shared by either the client or server.
    • Streams can be closed by either endpoint.
    • The order in which frames are sent on a stream is significant. Recipients process frames in the order they are received. In particular, the order of HEADERS, and DATA frames is semantically significant.
    • Streams are identified by an integer. Stream identifiers are assigned to streams by the endpoint initiating the stream.


    • 一个单独的HTTP/2连接能够保持多个同时打开的流,各个端点间从多个流中交换帧。
    • 流可以被被客户端或者服务端单方面建立使用或分享。
    • 流可以被任何一个连接终端关闭。
    • 在流内发送帧的顺序很重要。它们将按被接收的顺序处理。特别是报头及数据帧的顺序语义上是有意义的。
    • 流以一个整数标识。标识符有启动流的终端分配。

    5.1 Stream States 流状态

    The lifecycle of a stream is shown in Figure 1.


                     PP    |        |    PP
                  ,--------|  idle  |--------.
                 /         |        |         
                v          +--------+          v
         +----------+          |           +----------+
         |          |          | H         |          |
     ,---| reserved |          |           | reserved |---.
     |   | (local)  |          v           | (remote) |   |
     |   +----------+      +--------+      +----------+   |
     |      |          ES  |        |  ES          |      |
     |      | H    ,-------|  open  |-------.      | H    |
     |      |     /        |        |             |      |
     |      v    v         +--------+         v    v      |
     |   +----------+          |           +----------+   |
     |   |   half   |          |           |   half   |   |
     |   |  closed  |          | R         |  closed  |   |
     |   | (remote) |          |           | (local)  |   |
     |   +----------+          |           +----------+   |
     |        |                v                 |        |
     |        |  ES / R    +--------+  ES / R    |        |
     |        `----------->|        |<-----------'        |
     |  R                  | closed |                  R  |
     `-------------------->|        |<--------------------'
       H:  HEADERS frame (with implied CONTINUATIONs)
       PP: PUSH_PROMISE frame (with implied CONTINUATIONs)
       ES: END_STREAM flag
       R:  RST_STREAM frame

    $$Figure 2: Stream States$$

    Note that this diagram shows stream state transitions and frames that affect those transitions only. In this regard, CONTINUATION frames do not result in state transitions and are effectively part of the HEADERS or PUSH_PROMISE that they follow.


    Both endpoints have a subjective view of the state of a stream that could be different when frames are in transit. Endpoints do not coordinate the creation of streams; they are created unilaterally by either endpoint. The negative consequences of a mismatch in states are limited to the "closed" state after sending RST_STREAM, where frames might be received for some time after closing.



    Streams have the following states:

    idle :

    All streams start in the "idle" state. In this state, no frames have been exchanged.


    The following transitions are valid from this state:

    • Sending or receiving a HEADERS frame causes the stream to become "open". The stream identifier is selected as described in Section 5.1.1. The same HEADERS frame can also cause a stream to immediately become "half closed".
    • Sending a PUSH_PROMISE frame marks the associated stream for later use. The stream state for the reserved stream transitions to "reserved (local)".
    • Receiving a PUSH_PROMISE frame marks the associated stream as reserved by the remote peer. The state of the stream becomes "reserved (remote)".


    • 发送或者接收一个报头HEADERS帧导致流变成“打开”。流标识符如StreamIdentifiers说明。这个报头HEADERS帧同样可能导致流立即变成“半关闭”状态。
    • 发送一个推送承诺PUSH_PROMISE帧标记相关的流后续再使用。保留流状态将转换为“保留(本地)”。
    • 接收一个推送承诺PUSH_PROMISE帧标记相关的流为远程端点预留的流。这些流的状态变成“保留(远程)”

    reserved (local) :

    A stream in the "reserved (local)" state is one that has been promised by sending a PUSH_PROMISE frame. A PUSH_PROMISE frame reserves an idle stream by associating the stream with an open stream that was initiated by the remote peer (see Section 8.2).


    In this state, only the following transitions are possible:

    • The endpoint can send a HEADERS frame. This causes the stream to open in a "half closed (remote)" state.
    • Either endpoint can send a RST_STREAM frame to cause the stream to become "closed". This releases the stream reservation.


    • 端点可以发送报头HEADERS帧,致使流打开到“半封闭(远程)”状态。
    • 任意端点能发送一个RST_STREAM帧来使流变成“关闭”。这将释放流的保留。

    An endpoint MUST NOT send frames other than HEADERS or RST_STREAM in this state.


    A PRIORITY frame MAY be received in this state. Receiving any frames other than RST_STREAM, or PRIORITY MUST be treated as a connection error (Section 5.4.1) of type PROTOCOL_ERROR.


    reserved (remote) :

    A stream in the "reserved (remote)" state has been reserved by a remote peer.


    In this state, only the following transitions are possible:

    • Receiving a HEADERS frame causes the stream to transition to "half closed (local)".
    • Either endpoint can send a RST_STREAM frame to cause the stream to become "closed". This releases the stream reservation.


    • 接收一个报头HEADERS帧并致使流转换到“半封闭(本地)”状态。
    • 任意一个端点能发送一个RST_STREAM 帧来使流变成“关闭”。这将释放流的保留。

    An endpoint MAY send a PRIORITY frame in this state to reprioritize the reserved stream. An endpoint MUST NOT send any other type of frame other than RST_STREAM or PRIORITY.

    这种状态下任意终端可以发送一个优先级PRIORITY帧来变更保留流的优先级顺序。终端绝对不能发送任何RST_STREAM 和优先级PRIORITY以外的帧。

    Receiving any other type of frame other than HEADERS or RST_STREAM MUST be treated as a connection error (Section 5.4.1) of type PROTOCOL_ERROR.

    接收任何RST_STREAM 和优先级PRIORITY以外的帧必须作为类型为 协议错误PROTOCOL_ERROR的连接错误(章节5.4.1)来处理。

    open :

    A stream in the "open" state may be used by both peers to send frames of any type. In this state, sending peers observe advertised stream level flow control limits (Section 5.2).


    From this state either endpoint can send a frame with an END_STREAM flag set, which causes the stream to transition into one of the "half closed" states: an endpoint sending an END_STREAM flag causes the stream state to become "half closed (local)"; an endpoint receiving an END_STREAM flag causes the stream state to become "half closed (remote)".


    Either endpoint can send a RST_STREAM frame from this state, causing it to transition immediately to "closed".


    half closed (local) :

    A stream that is in the "half closed (local)" state cannot be used for sending frames. Only WINDOW_UPDATE, PRIORITY and RST_STREAM frames can be sent in this state.


    A stream transitions from this state to "closed" when a frame that contains an END_STREAM flag is received, or when either peer sends a RST_STREAM frame.


    A receiver can ignore WINDOW_UPDATE frames in this state, which might arrive for a short period after a frame bearing the END_STREAM flag is sent.


    PRIORITY frames received in this state are used to reprioritize streams that depend on the current stream.


    half closed (remote) :

    A stream that is "half closed (remote)" is no longer being used by the peer to send frames. In this state, an endpoint is no longer obligated to maintain a receiver flow control window if it performs flow control.


    If an endpoint receives additional frames for a stream that is in this state, other than WINDOW_UPDATE, PRIORITY or RST_STREAM, it MUST respond with a stream error (Section 5.4.2) of type STREAM_CLOSED.


    A stream can transition from this state to "closed" by sending a frame that contains an END_STREAM flag, or when either peer sends a RST_STREAM frame.


    closed :

    The "closed" state is the terminal state.


    An endpoint MUST NOT send frames on a closed stream. An endpoint that receives any frame other than PRIORITY after receiving a RST_STREAM MUST treat that as a stream error (Section 5.4.2) of type STREAM_CLOSED. Similarly, an endpoint that receives any frames after receiving a frame with the END_STREAM flag set MUST treat that as a connection error (Section 5.4.1) of type STREAM_CLOSED, unless the frame is permitted as described below.

    终端绝对不能通过关闭的流发送帧。终端在收到RST_STREAM后接收的任何帧必须作为类型为流关闭STREAM_CLOSED的StreamErrorHandler流错误stream error(章节5.4.2)处理。相似的,终端接收到带有END_STREAM标记设置的数据DATA帧之后的任何帧,或在带有END_STREAM终止流标记且后面没有延续CONTINUATION帧的报头HEADERS帧之后收到任何帧都必须作为类型为流关闭STREAM_CLOSED的连接错误(章节5.4.1)处理。

    WINDOW_UPDATE or RST_STREAM frames can be received in this state for a short period after a DATA or HEADERS frame containing an END_STREAM flag is sent. Until the remote peer receives and processes the frame bearing the END_STREAM flag, it might send frames of these types. Endpoints MUST ignore WINDOW_UPDATE or RST_STREAM frames received in this state, though endpoints MAY choose to treat frames that arrive a significant time after sending END_STREAM as a connection error (Section 5.4.1) of type PROTOCOL_ERROR.

    在这种情况下,在带有END_STREAM标记的DATA或HEADERS帧发送之后一小段时间内可以接收WINDOW_UPDATE或者RST_STREAM帧。在远端对等端接收并处理带有END_STREAM标记的帧之前,可以发送任意这几种帧。在这种状态下终端必须忽略接收到的WINDOW_UPDATE,PRIORITY, 或 RST_STREAM帧,但终端也可以当作类型为PROTOCOL_ERROR的连接错误(章节5.4.1)处理。

    PRIORITY frames can be sent on closed streams to prioritize streams that are dependent on the closed stream. Endpoints SHOULD process PRIORITY frame, though they can be ignored if the stream has been removed from the dependency tree (see Section 5.3.4).


    If this state is reached as a result of sending a RST_STREAM frame, the peer that receives the RST_STREAM might have already sent - or enqueued for sending - frames on the stream that cannot be withdrawn. An endpoint MUST ignore frames that it receives on closed streams after it has sent a RST_STREAM frame. An endpoint MAY choose to limit the period over which it ignores frames and treat frames that arrive after this time as being in error.


    Flow controlled frames (i.e., DATA) received after sending RST_STREAM are counted toward the connection flow control window. Even though these frames might be ignored, because they are sent before the sender receives the RST_STREAM, the sender will consider the frames to count against the flow control window.


    An endpoint might receive a PUSH_PROMISE frame after it sends RST_STREAM. PUSH_PROMISE causes a stream to become "reserved" even if the associated stream has been reset. Therefore, a RST_STREAM is needed to close an unwanted promised stream.


    In the absence of more specific guidance elsewhere in this document, implementations SHOULD treat the receipt of a message that is not expressly permitted in the description of a state as a connection error (Section 5.4.1) of type PROTOCOL_ERROR.


    5.1.1 Stream Identifiers 流标识

    Streams are identified with an unsigned 31-bit integer. Streams initiated by a client MUST use odd-numbered stream identifiers; those initiated by the server MUST use even-numbered stream identifiers. A stream identifier of zero (0x0) is used for connection control messages; the stream identifier zero cannot be used to establish a new stream.


    HTTP/1.1 requests that are upgraded to HTTP/2 (see Section 3.2) are responded to with a stream identifier of one (0x1). After the upgrade completes, stream 0x1 is "half closed (local)" to the client. Therefore, stream 0x1 cannot be selected as a new stream identifier by a client that upgrades from HTTP/1.1.


    The identifier of a newly established stream MUST be numerically greater than all streams that the initiating endpoint has opened or reserved. This governs streams that are opened using a HEADERS frame and streams that are reserved using PUSH_PROMISE. An endpoint that receives an unexpected stream identifier MUST respond with a connection error (Section 5.4.1) of type PROTOCOL_ERROR.


    The first use of a new stream identifier implicitly closes all streams in the "idle" state that might have been initiated by that peer with a lower-valued stream identifier. For example, if a client sends a HEADERS frame on stream 7 without ever sending a frame on stream 5, then stream 5 transitions to the "closed" state when the first frame for stream 7 is sent or received.


    Stream identifiers cannot be reused. Long-lived connections can result in an endpoint exhausting the available range of stream identifiers. A client that is unable to establish a new stream identifier can establish a new connection for new streams. A server that is unable to establish a new stream identifier can send a GOAWAY frame so that the client is forced to open a new connection for new streams.


    5.1.2 Stream Concurrency 流并发

    A peer can limit the number of concurrently active streams using the SETTINGS_MAX_CONCURRENT_STREAMS parameter (see Section 6.5.2) within a SETTINGS frame. The maximum concurrent streams setting is specific to each endpoint and applies only to the peer that receives the setting. That is, clients specify the maximum number of concurrent streams the server can initiate, and servers specify the maximum number of concurrent streams the client can initiate.


    Streams that are in the "open" state, or either of the "half closed" states count toward the maximum number of streams that an endpoint is permitted to open. Streams in any of these three states count toward the limit advertised in the SETTINGS_MAX_CONCURRENT_STREAMS setting. Streams in either of the "reserved" states do not count toward the stream limit.


    Endpoints MUST NOT exceed the limit set by their peer. An endpoint that receives a HEADERS frame that causes their advertised concurrent stream limit to be exceeded MUST treat this as a stream error (Section 5.4.2). An endpoint that wishes to reduce the value of SETTINGS_MAX_CONCURRENT_STREAMS to a value that is below the current number of open streams can either close streams that exceed the new value or allow streams to complete.


    5.2 Flow Control 流量控制

    Using streams for multiplexing introduces contention over use of the TCP connection, resulting in blocked streams. A flow control scheme ensures that streams on the same connection do not destructively interfere with each other. Flow control is used for both individual streams and for the connection as a whole.


    HTTP/2 provides for flow control through use of the WINDOW_UPDATE frame (Section 6.9).

    HTTP/2 通过使用WINDOW_UPDATE帧类型来提供流量控制(章节6.9)。

    5.2.1 Flow Control Principles 流量控制规则

    HTTP/2 stream flow control aims to allow for future improvements to flow control algorithms without requiring protocol changes. Flow control in HTTP/2 has the following characteristics:

    1. Flow control is hop-by-hop, not end-to-end.
    2. Flow control is based on window update frames. Receivers advertise how many bytes they are prepared to receive on a stream and for the entire connection. This is a credit-based scheme.
    3. Flow control is directional with overall control provided by the receiver. A receiver MAY choose to set any window size that it desires for each stream and for the entire connection. A sender MUST respect flow control limits imposed by a receiver. Clients, servers and intermediaries all independently advertise their flow control window as a receiver and abide by the flow control limits set by their peer when sending.
    4. The initial value for the flow control window is 65,535 bytes for both new streams and the overall connection.
    5. The frame type determines whether flow control applies to a frame. Of the frames specified in this document, only DATA frames are subject to flow control; all other frame types do not consume space in the advertised flow control window. This ensures that important control frames are not blocked by flow control.
    6. Flow control cannot be disabled.
    7. HTTP/2 defines only the format and semantics of the WINDOW_UPDATE frame (Section 6.9). This document does not stipulate how a receiver decides when to send this frame or the value that it sends. Nor does it specify how a sender chooses to send packets. Implementations are able to select any algorithm that suits their needs.


    1. 流量控制是逐跳的,而不是头尾连接的。
    2. 流量控制是基于窗口更新帧的。接收端广播自己准备在流及整个连接过程中接收的字节大小。这是一个信用为基础的方案。
    3. 流量控制是有方向性的,由接收端全权掌握。接收端可以选择针对流及整个连接设置任意的窗口大小。发送端必须遵守接收端的流量控制限制。客户端、服务端及中端代理作为接收者时都独立的向外广播他们各自的流量控制窗口,作为发送者时遵守接收端的限制。
    4. 每个新的流及整个连接的流量控制窗口初始值是65,535字节。
    5. 帧类型决定了是否适用流量控制规则。本文档定义的帧中,只有DATA帧受流量控制;所有其他的帧不受广播的流量控制窗口影响。这保证了重要的控制帧不因流量控制所阻塞。
    6. 流量控制不能被禁用。
    7. HTTP/2只标准化WINDOW_UPDATE帧格式(WINDOW_UPDATE)。它没有规定接收端是何时发送帧或者发送什么值,也没有规定发送端如何选择发送包。具体实现可以选择任何满足需求的算法。

    Implementations are also responsible for managing how requests and responses are sent based on priority; choosing how to avoid head of line blocking for requests; and managing the creation of new streams. Algorithm choices for these could interact with any flow control algorithm.


    5.2.2 Appropriate Use of Flow Control 正确使用流量控制

    Flow control is defined to protect endpoints that are operating under resource constraints. For example, a proxy needs to share memory between many connections, and also might have a slow upstream connection and a fast downstream one. Flow control addresses cases where the receiver is unable process data on one stream, yet wants to continue to process other streams in the same connection.


    Deployments that do not require this capability can advertise a flow control window of the maximum size, incrementing the available space when new data is received. This effectively disables flow control for that receiver. Conversely, a sender is always subject to the flow control window advertised by the receiver.


    Deployments with constrained resources (for example, memory) can employ flow control to limit the amount of memory a peer can consume. Note, however, that this can lead to suboptimal use of available network resources if flow control is enabled without knowledge of the bandwidth-delay product (see [RFC1323]).

    资源约束下(例如内存)的调度可以使用流量来限制一个对等端可以消耗的内存数量。需要注意的是如果在不知道带宽延迟乘积的时候启用流量控制可能导致无法最优的利用可用的网络资源(see RFC1323)。

    Even with full awareness of the current bandwidth-delay product, implementation of flow control can be difficult. When using flow control, the receiver MUST read from the TCP receive buffer in a timely fashion. Failure to do so could lead to a deadlock when critical frames, such as WINDOW_UPDATE, are not read and acted upon.


    5.3 Stream priority 流优先级

    A client can assign a priority for a new stream by including prioritization information in the HEADERS frame (Section 6.2) that opens the stream. For an existing stream, the PRIORITY frame (Section 6.3) can be used to change the priority.


    The purpose of prioritization is to allow an endpoint to express how it would prefer its peer allocate resources when managing concurrent streams. Most importantly, priority can be used to select streams for transmitting frames when there is limited capacity for sending.


    Explicitly setting the priority for a stream is input to a prioritization process. It does not guarantee any particular processing or transmission order for the stream relative to any other stream. An endpoint cannot force a peer to process concurrent streams in a particular order using priority. Expressing priority is therefore only ever a suggestion.


    Prioritization information can be specified explicitly for streams as they are created using the HEADERS frame, or changed using the PRIORITY frame. Providing prioritization information is optional, so default values are used if no explicit indicator is provided (Section 5.3.5).


    5.3.1 Stream Dependencies 流依赖

    Each stream can be given an explicit dependency on another stream. Including a dependency expresses a preference to allocate resources to the identified stream rather than to the dependent stream.


    A stream that is not dependent on any other stream is given a stream dependency of 0x0. In other words, the non-existent stream 0 forms the root of the tree.


    A stream that depends on another stream is a dependent stream. The stream upon which a stream is dependent is a parent stream. A dependency on a stream that is not currently in the tree - such as a stream in the "idle" state - results in the stream being given a default priority (Section 5.3.5).

    依赖其他流的流是一个有依赖流。被依赖的流是父节点流。被依赖的流如果当前不在依赖树中——例如处于“空闲”状态的流——流将会被赋予一个默认的优先级(章节 5.3.5)。

    When assigning a dependency on another stream, the stream is added as a new dependency of the parent stream. Dependent streams that share the same parent are not order with respect to each other. For example, if streams B and C are dependent on stream A, and if stream D is created with a dependency on stream A, this results in a dependency order of A followed by B, C, and D in any order.


        A                 A
       /       ==>      /|
      B   C             B D C

    $$Example of Default Dependency Creation$$

    An exclusive flag allows for the insertion of a new level of dependencies. The exclusive flag causes the stream to become the sole dependency of its parent stream, causing other dependencies to become dependent on the prioritized stream. In the previous example, if stream D is created with an exclusive dependency on stream A, this results in D becoming the dependency parent of B and C.


        A                 |
       /       ==>       D
      B   C              / 
                        B   C

    $$Example of Exclusive Dependency Creation$$

    Inside the dependency tree, a dependent stream SHOULD only be allocated resources if all of the streams that it depends on (the chain of parent streams up to 0x0) are either closed, or it is not possible to make progress on them.

    在一个依赖树中,一个有依赖的有应该只有在所有其依赖的父节点流(一直到流 0x0的所有父节点流)都关闭或者无法取得进展的情况下才能被分配资源。

    A stream cannot depend on itself. An endpoint MUST treat this as a stream error (Section 5.4.2) of type PROTOCOL_ERROR.


    5.3.2 Dependency Weighting 依赖权重

    All dependent streams are allocated an integer weight between 1 to 256 (inclusive).


    Streams with the same parent SHOULD be allocated resources proportionally based on their weight. Thus, if stream B depends on stream A with weight 4, and C depends on stream A with weight 12, and if no progress can be made on A, stream B ideally receives one third of the resources allocated to stream C.


    5.3.3 Reprioritization 优先级重组

    Stream priorities are changed using the PRIORITY frame. Setting a dependency causes a stream to become dependent on the identified parent stream.


    Dependent streams move with their parent stream if the parent is reprioritized. Setting a dependency with the exclusive flag for a reprioritized stream moves all the dependencies of the new parent stream to become dependent on the reprioritized stream.


    If a stream is made dependent on one of its own dependencies, the formerly dependent stream is first moved to be dependent on the reprioritized stream's previous parent. The moved dependency retains its weight.


    For example, consider an original dependency tree where B and C depend on A, D and E depend on C, and F depends on D. If A is made dependent on D, then D takes the place of A. All other dependency relationships stay the same, except for F, which becomes dependent on A if the reprioritization is exclusive.


        ?                ?                ?                 ?
        |               /                |                 |
        A              D   A              D                 D
       /             /   /             /                 |
      B   C     ==>  F   B   C   ==>    F   A       OR      A
         /                  |             /              /|
        D   E                E            B   C           B C F
        |                                     |             |
        F                                     E             E
                   (intermediate)   (non-exclusive)    (exclusive)

    5.3.4 Prioritization State Management 优先级状态管理

    When a stream is removed from the dependency tree, its dependencies can be moved to become dependent on the parent of the closed stream. The weights of new dependencies are recalculated by distributing the weight of the dependency of the closed stream proportionally based on the weights of its dependencies.


    Streams that are removed from the dependency tree cause some prioritization information to be lost. Resources are shared between streams with the same parent stream, which means that if a stream in that set closes or becomes blocked, any spare capacity allocated to a stream is distributed to the immediate neighbors of the stream. However, if the common dependency is removed from the tree, those streams share resources with streams at the next highest level.


    For example, assume streams A and B share a parent, and streams C and D both depend on stream A. Prior to the removal of stream A, if streams A and D are unable to proceed, then stream C receives all the resources dedicated to stream A. If stream A is removed from the tree, the weight of stream A is divided between streams C and D. If stream D is still unable to proceed, this results in stream C receiving a reduced proportion of resources. For equal starting weights, C receives one third, rather than one half, of available resources.


    It is possible for a stream to become closed while prioritization information that creates a dependency on that stream is in transit. If a stream identified in a dependency has had any associated priority information destroyed, then the dependent stream is instead assigned a default priority. This potentially creates suboptimal prioritization, since the stream could be given a priority that is higher than intended.


    To avoid these problems, an endpoint SHOULD retain stream prioritization state for a period after streams become closed. The longer state is retained, the lower the chance that streams are assigned incorrect or default priority values.


    This could create a large state burden for an endpoint, so this state MAY be limited. An endpoint MAY apply a fixed upper limit on the number of closed streams for which prioritization state is tracked to limit state exposure. The amount of additional state an endpoint maintains could be dependent on load; under high load, prioritization state can be discarded to limit resource commitments. In extreme cases, an endpoint could even discard prioritization state for active or reserved streams. If a fixed limit is applied, endpoints SHOULD maintain state for at least as many streams as allowed by their setting for SETTINGS_MAX_CONCURRENT_STREAMS.


    An endpoint receiving a PRIORITY frame that changes the priority of a closed stream SHOULD alter the dependencies of the streams that depend on it, if it has retained enough state to do so.


    5.4 Error Handling 错误处理

    HTTP/2 framing permits two classes of error:

    • An error condition that renders the entire connection unusable is a connection error.
    • An error in an individual stream is a stream error.

    A list of error codes is included in Section 7.


    • 使整个连接不可用的错误。
    • 单个流中出现的错误。


    5.4.1 Connection Error Handling 连接错误处理

    A connection error is any error which prevents further processing of the framing layer, or which corrupts any connection state.


    An endpoint that encounters a connection error SHOULD first send a GOAWAY frame (Section 6.8) with the stream identifier of the last stream that it successfully received from its peer. The GOAWAY frame includes an error code that indicates why the connection is terminating. After sending the GOAWAY frame, the endpoint MUST close the TCP connection.


    It is possible that the GOAWAY will not be reliably received by the receiving endpoint. In the event of a connection error, GOAWAY only provides a best effort attempt to communicate with the peer about why the connection is being terminated.


    An endpoint can end a connection at any time. In particular, an endpoint MAY choose to treat a stream error as a connection error. Endpoints SHOULD send a GOAWAY frame when ending a connection, providing that circumstances permit it.


    5.4.2 Stream Error Handling 流错误处理

    A stream error is an error related to a specific stream that does not affect processing of other streams.


    An endpoint that detects a stream error sends a RST_STREAM frame (Section 6.4) that contains the stream identifier of the stream where the error occurred. The RST_STREAM frame includes an error code that indicates the type of error.


    A RST_STREAM is the last frame that an endpoint can send on a stream. The peer that sends the RST_STREAM frame MUST be prepared to receive any frames that were sent or enqueued for sending by the remote peer. These frames can be ignored, except where they modify connection state (such as the state maintained for header compression (Section 4.3), or flow control).


    Normally, an endpoint SHOULD NOT send more than one RST_STREAM frame for any stream. However, an endpoint MAY send additional RST_STREAM frames if it receives frames on a closed stream after more than a round-trip time. This behavior is permitted to deal with misbehaving implementations.


    An endpoint MUST NOT send a RST_STREAM in response to an RST_STREAM frame, to avoid looping.


    .4.3 Connection Termination 连接终止

    If the TCP connection is torn down while streams remain in open or half closed states, then the endpoint MUST assume that those streams were abnormally interrupted and could be incomplete.


    .5 Extending HTTP/2 HTTP/2扩展

    HTTP/2 permits extension of the protocol. Protocol extensions can be used to provide additional services or alter any aspect of the protocol, within the limitations described in this section. Extensions are effective only within the scope of a single HTTP/2 connection.


    Extensions are permitted to use new frame types (Section 4.1), new settings (Section 6.5.2), new error codes (Section 7), or new header fields that start with a colon (:). Of these, registries are established for frame types (Section 11.2), settings (Section 11.3) and error codes (Section 11.4).


    Implementations MUST ignore unknown or unsupported values in all extensible protocol elements. Implementations MUST discard frames that have unknown or unsupported types. This means that any of these extension points can be safely used by extensions without prior arrangement or negotiation.


    However, extensions that could change the semantics of existing protocol components MUST be negotiated before being used. For example, an extension that changes the layout of the HEADERS frame cannot be used until the peer has given a positive signal that this is acceptable. In this case, it could also be necessary to coordinate when the revised layout comes into effect. Note that treating any frame other than DATA frames as flow controlled is such a change in semantics, and can only be done through negotiation.


    This document doesn't mandate a specific method for negotiating the use of an extension, but notes that a setting (Section 6.5.2) could be used for that purpose. If both peers set a value that indicates willingness to use the extension, then the extension can be used. If a setting is used for extension negotiation, the initial value MUST be defined so that the extension is initially disabled.


     Frame Definitions 帧定义

    This specification defines a number of frame types, each identified by a unique 8-bit type code. Each frame type serves a distinct purpose either in the establishment and management of the connection as a whole, or of individual streams.


    The transmission of specific frame types can alter the state of a connection. If endpoints fail to maintain a synchronized view of the connection state, successful communication within the connection will no longer be possible. Therefore, it is important that endpoints have a shared comprehension of how the state is affected by the use any given frame.


    .1 DATA 数据帧

    DATA frames (type=0x0) convey arbitrary, variable-length sequences of octets associated with a stream. One or more DATA frames are used, for instance, to carry HTTP request or response payloads.


    DATA frames MAY also contain arbitrary padding. Padding can be added to DATA frames to obscure the size of messages.


     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     | [Pad High(8)] | [Pad Low (8)] |          Data (*)             .
     .                            Data (*)                         ...
     |                           Padding (*)                       ...

    The DATA frame contains the following fields:

    • Pad Length:An 8-bit field containing the length of the frame padding in units of octets. This field is optional and is only present if the PADDED flag is set.
    • Data:Application data. The amount of data is the remainder of the frame payload after subtracting the length of the other fields that are present.
    • Padding:Padding octets that contain no application semantic value. Padding octets MUST be set to zero when sending and ignored when receiving.


    • Pad Length : 包含字节为单位的帧填充长度的8位字段。这个字段是可选的,并且只在设置了PADDED标记的情况下呈现。
    • Data : 应用数据。数据量的大小是帧的有效载荷减去其他呈现字段的长度。
    • Padding : 填充字节不包含任何应用语义值。填充字节必须在发送的时候设置为0,在接收的时候忽略。

    The DATA frame defines the following flags:

    • END_STREAM (0x1):Bit 1 being set indicates that this frame is the last that the endpoint will send for the identified stream. Setting this flag causes the stream to enter one of the "half closed" states or the "closed" state (Section 5.1).
    • END_SEGMENT (0x2):Bit 2 being set indicates that this frame is the last for the current segment. Intermediaries MUST NOT coalesce frames across a segment boundary and MUST preserve segment boundaries when forwarding frames.
    • PADDED (0x8):Bit 4 being set indicates that the Pad Length field is present.


    • END_STREAM (0x1) : 位1用来表示当前帧是确定的流发送的最后一帧。设置这个标记时流进入到一种半封闭状态或者关闭状态(章节5.1)。
    • END_SEGMENT (0x2) : 位2表示是当前端的最后一帧。代理端绝对不能跨越多个端的边界来合并帧,转发帧的时候代理端必须保持片段的边界。
    • PADDED (0x8) : 位4用来表示Pad Length 字段是可见的。

    DATA frames MUST be associated with a stream. If a DATA frame is received whose stream identifier field is 0x0, the recipient MUST respond with a connection error (Section 5.4.1) of type PROTOCOL_ERROR.


    DATA frames are subject to flow control and can only be sent when a stream is in the "open" or "half closed (remote)" states. The entire DATA frame payload is included in flow control, including Pad Length and Padding fields if present. If a DATA frame is received whose stream is not in "open" or "half closed (local)" state, the recipient MUST respond with a stream error (Section 5.4.2) of type STREAM_CLOSED.


    The total number of padding octets is determined by the value of the Pad Length field. If the length of the padding is greater than the length of the remainder of the frame payload, the recipient MUST treat this as a connection error (Section 5.4.1) of type PROTOCOL_ERROR.

    填充字节的总数取决于Pad Length 的值。如果填充物的大小大于帧有效载荷的大小,接收端必须作为类型为协议错误的连接错误(章节5.4.1)处理。

    Note: A frame can be increased in size by one octet by including a Pad Length field with a value of zero.

    请注意 : 为帧的大小加1字节可通过增加一个值为0的Pad Length 值的方法。

    Use of padding is a security feature; as such, its use demands some care, see Section 10.7.


    .2 HEADERS 报头

    The HEADERS frame (type=0x1) carries name-value pairs. It is used to open a stream (Section 5.1). HEADERS frames can be sent on a stream in the "open" or "half closed (remote)" states.


      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     |Pad Length? (8)|
     |E|                 Stream Dependency? (31)                     |
     |  Weight? (8)  |
     |                   Header Block Fragment (*)                 ...
     |                           Padding (*)                       ...

    The HEADERS frame payload has the following fields:

    • Pad Length:An 8-bit field containing the length of the frame padding in units of octets. This field is optional and is only present if the PADDED flag is set.
    • E:A single bit flag indicates that the stream dependency is exclusive, see Section 5.3. This field is optional and is only present if the PRIORITY flag is set.
    • Stream Dependency:A 31-bit stream identifier for the stream that this stream depends on, see Section 5.3. This field is optional and is only present if the PRIORITY flag is set.
    • Weight:An 8-bit weight for the stream, see Section 5.3. Add one to the value to obtain a weight between 1 and 256. This field is optional and is only present if the PRIORITY flag is set.
    • Header Block Fragment:A header block fragment (Section 4.3).
    • Padding:Padding octets.


    • Pad Length : 8位的包含单位为字节帧填充长度字段。这个字段是可选的并只有在设置了PADDED 标记的情况下才呈现。
    • E : 1位的标记用于标识流依赖是否是专用的,见章节5.3。这个字段是可选的,并且只在优先级标记设置的情况下才呈现。
    • Stream Dependency : 31位流所依赖的流的标识符的字段,见章节5.3。这个字段是可选的,并且只在优先级标记设置的情况下才呈现。
    • Weight : 流的8位权重标记,见章节5.3。添加一个1-256的值来存储流的权重。这个字段是可选的,并且只在优先级标记设置的情况下才呈现。
    • Header Block Fragment : 报头块碎片。
    • Padding : 填充字节

    The HEADERS frame defines the following flags:

    • END_STREAM (0x1):Bit 1 being set indicates that the header block (Section 4.3) is the last that the endpoint will send for the identified stream. Setting this flag causes the stream to enter one of "half closed" states (Section 5.1).
    • A HEADERS frame that is followed by CONTINUATION frames carries the END_STREAM flag that signals the end of a stream. A CONTINUATION frame cannot be used to terminate a stream.
    • END_SEGMENT (0x2):Bit 2 being set indicates that this frame is the last for the current segment. Intermediaries MUST NOT coalesce frames across a segment boundary and MUST preserve segment boundaries when forwarding frames. END_SEGMENT, like END_STREAM, applies to a complete sequence of HEADERS and CONTINUATION frames.
    • END_HEADERS (0x4):Bit 3 being set indicates that this frame contains an entire header block (Section 4.3) and is not followed by any CONTINUATION frames. A HEADERS frame without the END_HEADERS flag set MUST be followed by a CONTINUATION frame for the same stream. A receiver MUST treat the receipt of any other type of frame or a frame on a different stream as a connection error (Section 5.4.1) of type PROTOCOL_ERROR.
    • PADDED (0x8):Bit 4 being set indicates that the Pad Length field is present.
    • PRIORITY (0x20):Bit 6 being set indicates that the Exclusive Flag (E), Stream Dependency, and Weight fields are present; see Section 5.3.


    • END_STREAM (0x1) : 位1用来标识这是发送端对确定的流发送的最后报头区块(章节4.3)。设置这个标记将使流进入一种半封闭状态(章节5.1)。 后面伴随带有END_STREAM标记的延续帧的报头帧表示流的终止。延续帧不用来用终止流。
    • END_SEGMENT (0x2) : 位2表示这是当前端的最后一帧。中介端绝对不能跨片段来合并帧,且在转发帧的时候必须保持片段的边界。
    • END_HEADERS (0x4) : 位3表示帧包含了整个的报头块(章节4.3),且后面没有延续帧。 不带有END_HEADERS标记的报头帧在同个流上后面必须跟着延续帧。接收端接收到任何其他类型的帧或者在其他流上的帧必须作为类型为协议错误的连接错误处理。
    • PADDED (0x8) : 位4表示Pad Length字段会呈现。
    • PRIORITY (0x8) : 位6设置指示专用标记(E),流依赖及权重字段将会呈现;见章节5.3

    The payload of a HEADERS frame contains a header block fragment (Section 4.3). A header block that does not fit within a HEADERS frame is continued in a CONTINUATION frame (Section 6.10).


    HEADERS frames MUST be associated with a stream. If a HEADERS frame is received whose stream identifier field is 0x0, the recipient MUST respond with a connection error (Section 5.4.1) of type PROTOCOL_ERROR.


    The HEADERS frame changes the connection state as described in Section 4.3.


    The HEADERS frame includes optional padding. Padding fields and flags are identical to those defined for DATA frames (Section 6.1).


    .3 PRIORITY 优先级帧

    The PRIORITY frame (type=0x2) specifies the sender-advised priority of a stream (Section 5.3). It can be sent at any time for an existing stream, including closed streams. This enables reprioritization of existing streams.


      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     |E|                  Stream Dependency (31)                     |
     |   Weight (8)  |

    The payload of a PRIORITY frame contains the following fields:

    • E: A single bit flag indicates that the stream dependency is exclusive, see Section 5.3.
    • Stream Dependency: A 31-bit stream identifier for the stream that this stream depends on, see Section 5.3.
    • Weight: An 8-bit weight for the identified stream dependency, see Section 5.3. Add one to the value to obtain a weight between 1 and 256.


    • E : 一位的标记,指示流的依赖是专有的,见章节5.3
    • Stream Dependency : 标识流所依赖的流的31位流标识符,见章节5.3
    • Weight: 流的依赖的的权重(8位),见章节5.3。添加一个1-256的权重值。

    The PRIORITY frame does not define any flags.


    The PRIORITY frame is associated with an existing stream. If a PRIORITY frame is received with a stream identifier of 0x0, the recipient MUST respond with a connection error (Section 5.4.1) of type PROTOCOL_ERROR.


    The PRIORITY frame can be sent on a stream in any of the "reserved (remote)", "open", "half closed (local)", "half closed (remote)", or "closed" states, though it cannot be sent between consecutive frames that comprise a single header block (Section 4.3). Note that this frame could arrive after processing or frame sending has completed, which would cause it to have no effect on the current stream. For a stream that is in the "half closed (remote)" or "closed" - state, this frame can only affect processing of the current stream and not frame transmission.


    The PRIORITY frame is the only frame that can be sent for a stream in the "closed" state. This allows for the reprioritization of a group of dependent streams by altering the priority of a parent stream, which might be closed. However, a PRIORITY frame sent on a closed stream risks being ignored due to the peer having discarded priority state information for that stream.



    The RST_STREAM frame (type=0x3) allows for abnormal termination of a stream. When sent by the initiator of a stream, it indicates that they wish to cancel the stream or that an error condition has occurred. When sent by the receiver of a stream, it indicates that either the receiver is rejecting the stream, requesting that the stream be cancelled, or that an error condition has occurred.


     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     |                        Error Code (32)                        |

    The RST_STREAM frame contains a single unsigned, 32-bit integer identifying the error code (Section 7). The error code indicates why the stream is being terminated.

    RST_STREAM 帧由一个无符号的32位整数标记错误码。错误码指明流被终止的原因。

    The RST_STREAM frame does not define any flags.

    RST_STREAM 帧未定义任何标记。

    The RST_STREAM frame fully terminates the referenced stream and causes it to enter the closed state. After receiving a RST_STREAM on a stream, the receiver MUST NOT send additional frames for that stream. However, after sending the RST_STREAM, the sending endpoint MUST be prepared to receive and process additional frames sent on the stream that might have been sent by the peer prior to the arrival of the RST_STREAM.

    RST_STREAM 帧完全终止相关的流并使其转入关闭状态。在接收到流的RST_STREAM帧后,接收端绝对不能在流上发送额外的帧。然而,在发送RST_STREAM帧后,发送端必须要准备接收并处理流上的其他帧,因为对等端有可能在收到RST_STREAM帧前就已经发送这些帧。

    RST_STREAM frames MUST be associated with a stream. If a RST_STREAM frame is received with a stream identifier of 0x0, the recipient MUST treat this as a connection error (Section 5.4.1) of type PROTOCOL_ERROR.

    RST_STREAM 帧必须与流相关联。如果接收端收到流标示符为0x0的RST_STREAM 帧,必须作为类型为协议错误的连接错误(章节5.4.1)处理。

    RST_STREAM frames MUST NOT be sent for a stream in the "idle" state. If a RST_STREAM frame identifying an idle stream is received, the recipient MUST treat this as a connection error (Section 5.4.1) of type PROTOCOL_ERROR.


    .5 SETTINGS 设置帧

    The SETTINGS frame (type=0x4) conveys configuration parameters that affect how endpoints communicate, such as preferences and constraints on peer behavior. The SETTINGS frame is also used to acknowledge the receipt of those parameters. Individually, a SETTINGS parameter can also be referred to as a "setting".


    SETTINGS parameters are not negotiated; they describe characteristics of the sending peer, which are used by the receiving peer. Different values for the same parameter can be advertised by each peer. For example, a client might set a high initial flow control window, whereas a server might set a lower value to conserve resources.


    A SETTINGS frame MUST be sent by both endpoints at the start of a connection, and MAY be sent at any other time by either endpoint over the lifetime of the connection. Implementations MUST support all of the parameters defined by this specification.


    Each parameter in a SETTINGS frame replaces any existing value for that parameter. Parameters are processed in the order in which they appear, and a receiver of a SETTINGS frame does not need to maintain any state other than the current value of its parameters. Therefore, the value of a SETTINGS parameter is the last value that is seen by a receiver.


    SETTINGS parameters are acknowledged by the receiving peer. To enable this, the SETTINGS frame defines the following flag:


    ACK (0x1) : Bit 1 being set indicates that this frame acknowledges receipt and application of the peer's SETTINGS frame. When this bit is set, the payload of the SETTINGS frame MUST be empty. Receipt of a SETTINGS frame with the ACK flag set and a length field value other than 0 MUST be treated as a connection error (Section 5.4.1) of type FRAME_SIZE_ERROR. For more info, see Settings Synchronization (Section 6.5.3).

    ACK (0x1) : 位1表示设置帧已被接收端接收并应用。如果这个位设置了,设置帧的载体必须为空。接收到字段长度不是0的带有ACK标记的设置帧必须作为类型为帧大小错误的连接错误(章节5.4.1)处理,更多信息,见同步设置(章节6.5.3)。

    SETTINGS frames always apply to a connection, never a single stream. The stream identifier for a SETTINGS frame MUST be zero (0x0). If an endpoint receives a SETTINGS frame whose stream identifier field is anything other than 0x0, the endpoint MUST respond with a connection error (Section 5.4.1) of type PROTOCOL_ERROR.


    The SETTINGS frame affects connection state. A badly formed or incomplete SETTINGS frame MUST be treated as a connection error (Section 5.4.1) of type PROTOCOL_ERROR.


    .5.1 SettingFormat 设置帧格式

    The payload of a SETTINGS frame consists of zero or more parameters, each consisting of an unsigned 16-bit setting identifier and an unsigned 32-bit value.


     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     |Identifier (8) |                 Value (32)                  ...
     ...Value        |

    .5.2 Defined SETTINGS Parameters 设置帧参数

    The following parameters are defined:

    • SETTINGS_HEADER_TABLE_SIZE (0x1):Allows the sender to inform the remote endpoint of the maximum size of the header compression table used to decode header blocks. The encoder can select any size equal to or less than this value by using signaling specific to the header compression format inside a header block. The initial value is 4,096 bytes.
    • SETTINGS_ENABLE_PUSH (0x2):This setting can be use to disable server push (Section 8.2). An endpoint MUST NOT send a PUSH_PROMISE frame if it receives this parameter set to a value of 0. An endpoint that has both set this parameter to 0 and had it acknowledged MUST treat the receipt of a PUSH_PROMISE frame as a connection error (Section 5.4.1) of type PROTOCOL_ERROR. The initial value is 1, which indicates that server push is permitted. Any value other than 0 or 1 MUST be treated as a connection error (Section 5.4.1) of type PROTOCOL_ERROR.
    • SETTINGS_MAX_CONCURRENT_STREAMS (0x3):Indicates the maximum number of concurrent streams that the sender will allow. This limit is directional: it applies to the number of streams that the sender permits the receiver to create. Initially there is no limit to this value. It is recommended that this value be no smaller than 100, so as to not unnecessarily limit parallelism. A value of 0 for SETTINGS_MAX_CONCURRENT_STREAMS SHOULD NOT be treated as special by endpoints. A zero value does prevent the creation of new streams, however this can also happen for any limit that is exhausted with active streams. Servers SHOULD only set a zero value for short durations; if a server does not wish to accept requests, closing the connection could be preferable.
    • SETTINGS_INITIAL_WINDOW_SIZE (0x4):Indicates the sender's initial window size (in bytes) for stream level flow control. The initial value is 65,535. This setting affects the window size of all streams, including existing streams, see Section 6.9.2. Values above the maximum flow control window size of 231 - 1 MUST be treated as a connection error (Section 5.4.1) of type FLOW_CONTROL_ERROR.


    • SETTINGS_HEADER_TABLE_SIZE (1) : 允许发送端通知远端终端解码报头区块的报头压缩表的最大承载量。这个编码器可以选择在报头区块中使用特定信号来减少报头压缩的大小(???)。初始值是4,096个字节。
    • SETTINGS_ENABLE_PUSH (2) : 这个参数可以用来关闭服务器推送。终端在接收到此参数为0的情况下绝对不能发送服务器推送承诺帧。终端在已经设置此参数为0并且承认的情况下必须对接收到的服务器推送作为类型为协议错误的连接错误处理。 初始值是1,表示推送是许可的。任何不是0或1的值必须作为类型为协议错误的连接错误处理。
    • SETTINGS_MAX_CONCURRENT_STREAMS (3) : 标明发送者允许的最大并发流。此限制是定向的:它适用于发送端允许接收端创建的最大并发流的数量。初始化时这个值没有限制。建议值不要大于100,以免不必要的限制并行。 此设置为0的值不应该被终端认为是特殊的。0的值阻止了新的流的创建,另外它也适用于被激活的流用尽的任何限制。对于短连接不应该设置此参数为0;如果服务端不希望接收任何请求,最佳的做法是关闭连接。
    • SETTINGS_INITIAL_WINDOW_SIZE (4) : 表示发送端对流层流量控制的初始窗口大小(字节单位)。初始值是65,535。 这个参数影响了所有流的窗口大小,包括现有的流。见章节6.9.2. 流量控制窗口大小值大于2的31次方-1的必须被作为流量控制错误的连接错误处理。

    An endpoint that receives a SETTINGS frame with any unknown or unsupported identifier MUST ignore that setting.


    .5.3 Settings Synchronization 设置同步

    Most values in SETTINGS benefit from or require an understanding of when the peer has received and applied the changed the communicated parameter values. In order to provide such synchronization timepoints, the recipient of a SETTINGS frame in which the ACK flag is not set MUST apply the updated parameters as soon as possible upon receipt.


    The values in the SETTINGS frame MUST be applied in the order they appear, with no other frame processing between values. Once all values have been applied, the recipient MUST immediately emit a SETTINGS frame with the ACK flag set. Upon receiving a SETTINGS frame with the ACK flag set, the sender of the altered parameters can rely upon their application.


    If the sender of a SETTINGS frame does not receive an acknowledgement within a reasonable amount of time, it MAY issue a connection error (Section 5.4.1) of type SETTINGS_TIMEOUT.


    .6 PUSH_PROMISE 推送承诺帧

    The PUSH_PROMISE frame (type=0x5) is used to notify the peer endpoint in advance of streams the sender intends to initiate. The PUSH_PROMISE frame includes the unsigned 31-bit identifier of the stream the endpoint plans to create along with a set of headers that provide additional context for the stream. Section 8.2 contains a thorough description of the use of PUSH_PROMISE frames.


     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     | [Pad High(8)] | [Pad Low (8)] |X|  Promised Stream ID (31)  ...
     ...    Promised Stream ID       | Header Block Fragment (*)   ...
     |                   Header Block Fragment (*)                 ...
     |                           Padding (*)                       ...

    The PUSH_PROMISE frame payload has the following fields:

    • Pad Length:An 8-bit field containing the length of the frame padding in units of octets. This field is optional and is only present if the PADDED flag is set.
    • R:A single reserved bit.
    • Promised Stream ID:This unsigned 31-bit integer identifies the stream the endpoint intends to start sending frames for. The promised stream identifier MUST be a valid choice for the next stream sent by the sender (see new stream identifier (Section 5.1.1)).
    • Header Block Fragment: header block fragment (Section 4.3) containing request header fields.
    • Padding:Padding octets.


    • Pad High : 填充大小高位。这个字段只有在PAD_HIGH标记设置的情况下才呈现。
    • Pad Low : 填充大小低位。这个字段只有在PAD_LOW标记设置的情况下才呈现。
    • X : 单独的保留位。
    • Promised Stream ID : 这个无符号31位整数表示终端准备发送的流标记。被承诺的流标记必须对发送端准备发送的下一个流来说是有效选择。
    • Header Block Fragment : 包含请求头字段的报头区块碎片(章节5.1.1)。
    • Padding : 填充字节。

    The PUSH_PROMISE frame defines the following flags:

    • END_HEADERS (0x4): Bit 3 being set indicates that this frame contains an entire header block (Section 4.3) and is not followed by any CONTINUATION frames. A PUSH_PROMISE frame without the END_HEADERS flag set MUST be followed by a CONTINUATION frame for the same stream. A receiver MUST treat the receipt of any other type of frame or a frame on a different stream as a connection error (Section 5.4.1) of type PROTOCOL_ERROR.
    • PADDED (0x8): Bit 4 being set indicates that the Pad Length field is present.


    • END_HEADERS (0x4) : 位3 表明帧包含了整个报头区块(章节4.3)并且不跟着延续帧。 不带有END_HEADERS标记的推送承诺帧在同个流上面后面必须跟着延续帧。接收端接收到任何其他类型或者其他流觞的帧必须作为类型为协议错误的连接错误(章节5.4.1)处理。
    • PADDED (0x8) : 位4 表明Pad长度字段是已设置。

    PUSH_PROMISE frames MUST be associated with an existing, peer-initiated stream. The stream identifier of a PUSH_PROMISE frame indicates the stream it is associated with. If the stream identifier field specifies the value 0x0, a recipient MUST respond with a connection error (Section 5.4.1) of type PROTOCOL_ERROR.


    Promised streams are not required to be used in the order they are promised. The PUSH_PROMISE only reserves stream identifiers for later use.


    PUSH_PROMISE MUST NOT be sent if the SETTINGS_ENABLE_PUSH setting of the peer endpoint is set to 0. An endpoint that has set this setting and has received acknowledgement MUST treat the receipt of a PUSH_PROMISE frame as a connection error (Section 5.4.1) of type PROTOCOL_ERROR.


    Recipients of PUSH_PROMISE frames can choose to reject promised streams by returning a RST_STREAM referencing the promised stream identifier back to the sender of the PUSH_PROMISE.


    A PUSH_PROMISE frame modifies the connection state in two ways. The inclusion of a header block (Section 4.3) potentially modifies the state maintained for header compression. PUSH_PROMISE also reserves a stream for later use, causing the promised stream to enter the "reserved" state. A sender MUST NOT send a PUSH_PROMISE on a stream unless that stream is either "open" or "half closed (remote)"; the sender MUST ensure that the promised stream is a valid choice for a new stream identifier (Section 5.1.1) (that is, the promised stream MUST be in the "idle" state).


    Since PUSH_PROMISE reserves a stream, ignoring a PUSH_PROMISE frame causes the stream state to become indeterminate. A receiver MUST treat the receipt of a PUSH_PROMISE on a stream that is neither "open" nor "half closed (local)" as a connection error (Section 5.4.1) of type PROTOCOL_ERROR. Similarly, a receiver MUST treat the receipt of a PUSH_PROMISE that promises an illegal stream identifier (Section 5.1.1) (that is, an identifier for a stream that is not currently in the "idle" state) as a connection error (Section 5.4.1) of type PROTOCOL_ERROR.

    由于PUSH_PROMISE保留了一个流、忽略一个PUSH_PROMISE 帧都会导致流状态变得不确定。接收端接收到流状态不是“打开”或者“半封闭(本地)”的流的推送承诺帧必须作为类型为协议错误的连接错误(章节5.4.1)处理。相似的,接收端必须对在一个非法标示(章节5.1.1)的流(即流的标识当前不在空闲状态)上建立的推送承诺作为类型为协议错误的连接错误(章节5.4.1)处理。

    The PUSH_PROMISE frame includes optional padding. Padding fields and flags are identical to those defined for DATA frames (Section 6.1).


    .7 PING PING帧

    The PING frame (type=0x6) is a mechanism for measuring a minimal round trip time from the sender, as well as determining whether an idle connection is still functional. PING frames can be sent from any endpoint.


     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     |                                                               |
     |                      Opaque Data (64)                         |
     |                                                               |

    In addition to the frame header, PING frames MUST contain 8 octets of data in the payload. A sender can include any value it chooses and use those bytes in any fashion.


    Receivers of a PING frame that does not include an ACK flag MUST send a PING frame with the ACK flag set in response, with an identical payload. PING responses SHOULD be given higher priority than any other frame.


    The PING frame defines the following flags:

    • ACK (0x1):Bit 1 being set indicates that this PING frame is a PING response. An endpoint MUST set this flag in PING responses. An endpoint MUST NOT respond to PING frames containing this flag.

      PING frames are not associated with any individual stream. If a PING frame is received with a stream identifier field value other than 0x0, the recipient MUST respond with a connection error (Section 5.4.1) of type PROTOCOL_ERROR.


    ACK (0x1) : 位1表示PING帧是一个PING响应。终端必须在PING响应中设置此标记。终端绝对不能对包含此标记的PING帧做出响应。


    Receipt of a PING frame with a length field value other than 8 MUST be treated as a connection error (Section 5.4.1) of type FRAME_SIZE_ERROR.


    .8 GOAWAY 超时帧

    The GOAWAY frame (type=0x7) informs the remote peer to stop creating streams on this connection. GOAWAY can be sent by either the client or the server. Once sent, the sender will ignore frames sent on any new streams with identifiers higher than the included last stream identifier. Receivers of a GOAWAY frame MUST NOT open additional streams on the connection, although a new connection can be established for new streams.


    The purpose of this frame is to allow an endpoint to gracefully stop accepting new streams, while still finishing processing of previously established streams. This enables administrative actions, like server maintainence.


    There is an inherent race condition between an endpoint starting new streams and the remote sending a GOAWAY frame. To deal with this case, the GOAWAY contains the stream identifier of the last stream which was or might be processed on the sending endpoint in this connection. If the receiver of the GOAWAY has sent data on streams with a higher stream identifier than what is indicated in the GOAWAY frame, those streams are not or will not be processed. The receiver of the GOAWAY frame can treat the streams as though they had never been created at all, thereby allowing those streams to be retried later on a new connection.


    Endpoints SHOULD always send a GOAWAY frame before closing a connection so that the remote can know whether a stream has been partially processed or not. For example, if an HTTP client sends a POST at the same time that a server closes a connection, the client cannot know if the server started to process that POST request if the server does not send a GOAWAY frame to indicate what streams it might have acted on.


    An endpoint might choose to close a connection without sending GOAWAY for misbehaving peers.


     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     |X|                  Last-Stream-ID (31)                        |
     |                      Error Code (32)                          |
     |                  Additional Debug Data (*)                    |

    The GOAWAY frame does not define any flags.


    The GOAWAY frame applies to the connection, not a specific stream. An endpoint MUST treat a GOAWAY frame with a stream identifier other than 0x0 as a connection error (Section 5.4.1) of type PROTOCOL_ERROR.


    The last stream identifier in the GOAWAY frame contains the highest numbered stream identifier for which the sender of the GOAWAY frame might have taken some action on, or might yet take action on. All streams up to and including the identified stream might have been processed in some way. The last stream identifier can be set to 0 if no streams were processed.


    Note:In this context, "processed" means that some data from the stream was passed to some higher layer of software that might have taken some action as a result.


    If a connection terminates without a GOAWAY frame, the last stream identifier is effectively the highest possible stream identifier.


    On streams with lower or equal numbered identifiers that were not closed completely prior to the connection being closed, re-attempting requests, transactions, or any protocol activity is not possible, with the exception of idempotent actions like HTTP GET, PUT, or DELETE. Any protocol activity that uses higher numbered streams can be safely retried using a new connection.

    连接关闭前小于或等于标识符上的流没有完全关闭的,重试请求,交换,或者任何协议活动都是不可能的(例如HTTP GET,PUT,或者删除等等幂行为例外)。任何使用更高的流数值的协议行为可以在新的连接上安全地重试。

    Activity on streams numbered lower or equal to the last stream identifier might still complete successfully. The sender of a GOAWAY frame might gracefully shut down a connection by sending a GOAWAY frame, maintaining the connection in an open state until all in-progress streams complete.


    An endpoint MAY send multiple GOAWAY frames if circumstances change. For instance, an endpoint that sends GOAWAY with NO_ERROR during graceful shutdown could subsequently encounter an condition that requires immediate termination of the connection. The last stream identifier from the last GOAWAY frame received indicates which streams could have been acted upon. Endpoints MUST NOT increase the value they send in the last stream identifier, since the peers might already have retried unprocessed requests on another connection.


    A client that is unable to retry requests loses all requests that are in flight when the server closes the connection. This is especially true for intermediaries that might not be serving clients using HTTP/2. A server that is attempting to gracefully shut down a connection SHOULD send an initial GOAWAY frame with the last stream identifier set to 231-1 and a NO_ERROR code. This signals to the client that a shutdown is imminent and that no further requests can be initiated. After waiting at least one round trip time, the server can send another GOAWAY frame with an updated last stream identifier. This ensures that a connection can be cleanly shut down without losing requests.

    服务端关闭连接时,终端无法重试请求的将丢失所有正在发送的请求。尤其是针对中介端无法使用HTTP/2服务客户端的时候。无负担尝试优雅关闭连接时应当发送一个携带2^{31} -1 大小的流标识符和一个错误码的初始超时帧。这个信号对客户端来说意味着即将关闭连接并且不能建立更多请求了。在等待至少一个RTT时间之后,服务端能发送另外一个带有更新后的最终流标识符超时帧。这个确保了连接能彻底的关闭而不用丢失请求。

    After sending a GOAWAY frame, the sender can discard frames for streams with identifiers higher than the identified last stream. However, any frames that alter connection state cannot be completely ignored. For instance, HEADERS, PUSH_PROMISE and CONTINUATION frames MUST be minimally processed to ensure the state maintained for header compression is consistent (see Section 4.3); similarly DATA frames MUST be counted toward the connection flow control window. Failure to process these frames can cause flow control or header compression state to become unsynchronized.


    The GOAWAY frame also contains a 32-bit error code (Section 7) that contains the reason for closing the connection.


    Endpoints MAY append opaque data to the payload of any GOAWAY frame. Additional debug data is intended for diagnostic purposes only and carries no semantic value. Debug information could contain security- or privacy-sensitive data. Logged or otherwise persistently stored debug data MUST have adequate safeguards to prevent unauthorized access.


    .9 WINDOW_UPDATE 窗口更新帧

    The WINDOW_UPDATE frame (type=0x8) is used to implement flow control; see Section 5.2 for an overview.


    Flow control operates at two levels: on each individual stream and on the entire connection.


    Both types of flow control are hop-by-hop; that is, only between the two endpoints. Intermediaries do not forward WINDOW_UPDATE frames between dependent connections. However, throttling of data transfer by any receiver can indirectly cause the propagation of flow control information toward the original sender.


    Flow control only applies to frames that are identified as being subject to flow control. Of the frame types defined in this document, this includes only DATA frames. Frames that are exempt from flow control MUST be accepted and processed, unless the receiver is unable to assign resources to handling the frame. A receiver MAY respond with a stream error (Section 5.4.2) or connection error (Section 5.4.1) of type FLOW_CONTROL_ERROR if it is unable to accept a frame.


     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     |X|              Window Size Increment (31)                     |

    The payload of a WINDOW_UPDATE frame is one reserved bit, plus an unsigned 31-bit integer indicating the number of bytes that the sender can transmit in addition to the existing flow control window. The legal range for the increment to the flow control window is 1 to 231 - 1 (0x7fffffff) bytes.

    WINDOW_UPDATE帧的载体是一个保留字节,加上一个无符号31为整数表明发送端除了现有的流量控制窗口可以发送的字节数。留空控制窗口有效的增量范围是$1 至 2^{31}-1$(0x7fffffff) 字节。

    The WINDOW_UPDATE frame does not define any flags.


    The WINDOW_UPDATE frame can be specific to a stream or to the entire connection. In the former case, the frame's stream identifier indicates the affected stream; in the latter, the value "0" indicates that the entire connection is the subject of the frame.


    WINDOW_UPDATE can be sent by a peer that has sent a frame bearing the END_STREAM flag. This means that a receiver could receive a WINDOW_UPDATE frame on a "half closed (remote)" or "closed" stream. A receiver MUST NOT treat this as an error, see Section 5.1.


    A receiver that receives a flow controlled frame MUST always account for its contribution against the connection flow control window, unless the receiver treats this as a connection error (Section 5.4.1). This is necessary even if the frame is in error. Since the sender counts the frame toward the flow control window, if the receiver does not, the flow control window at sender and receiver can become different.


    .9.1 The Flow Control Window 流量控制窗口

    Flow control in HTTP/2 is implemented using a window kept by each sender on every stream. The flow control window is a simple integer value that indicates how many bytes of data the sender is permitted to transmit; as such, its size is a measure of the buffering capacity of the receiver.


    Two flow control windows are applicable: the stream flow control window and the connection flow control window. The sender MUST NOT send a flow controlled frame with a length that exceeds the space available in either of the flow control windows advertised by the receiver. Frames with zero length with the END_STREAM flag set (that is, an empty DATA frame) MAY be sent if there is no available space in either flow control window.


    For flow control calculations, the 8 byte frame header is not counted.


    After sending a flow controlled frame, the sender reduces the space available in both windows by the length of the transmitted frame.


    The receiver of a frame sends a WINDOW_UPDATE frame as it consumes data and frees up space in flow control windows. Separate WINDOW_UPDATE frames are sent for the stream and connection level flow control windows.


    A sender that receives a WINDOW_UPDATE frame updates the corresponding window by the amount specified in the frame.


    A sender MUST NOT allow a flow control window to exceed 231 - 1 bytes. If a sender receives a WINDOW_UPDATE that causes a flow control window to exceed this maximum it MUST terminate either the stream or the connection, as appropriate. For streams, the sender sends a RST_STREAM with the error code of FLOW_CONTROL_ERROR code; for the connection, a GOAWAY frame with a FLOW_CONTROL_ERROR code.


    Flow controlled frames from the sender and WINDOW_UPDATE frames from the receiver are completely asynchronous with respect to each other. This property allows a receiver to aggressively update the window size kept by the sender to prevent streams from stalling.


    .9.2 Initial Flow Control Window Size 流量控制窗口初始值

    When an HTTP/2 connection is first established, new streams are created with an initial flow control window size of 65,535 bytes. The connection flow control window is 65,535 bytes. Both endpoints can adjust the initial window size for new streams by including a value for SETTINGS_INITIAL_WINDOW_SIZE in the SETTINGS frame that forms part of the connection preface. The connection flow control window can only be changed using WINDOW_UPDATE frames.


    Prior to receiving a SETTINGS frame that sets a value for SETTINGS_INITIAL_WINDOW_SIZE, an endpoint can only use the default initial window size when sending flow controlled frames. Similarly, the connection flow control window is set to the default initial window size until a WINDOW_UPDATE frame is received.


    A SETTINGS frame can alter the initial flow control window size for all current streams. When the value of SETTINGS_INITIAL_WINDOW_SIZE changes, a receiver MUST adjust the size of all stream flow control windows that it maintains by the difference between the new value and the old value.


    A change to SETTINGS_INITIAL_WINDOW_SIZE can cause the available space in a flow control window to become negative. A sender MUST track the negative flow control window, and MUST NOT send new flow controlled frames until it receives WINDOW_UPDATE frames that cause the flow control window to become positive.


    For example, if the client sends 60KB immediately on connection establishment, and the server sets the initial window size to be 16KB, the client will recalculate the available flow control window to be -44KB on receipt of the SETTINGS frame. The client retains a negative flow control window until WINDOW_UPDATE frames restore the window to being positive, after which the client can resume sending.


    A SETTINGS frame cannot alter the connection flow control window.


    An endpoint MUST treat a change to SETTINGS_INITIAL_WINDOW_SIZE that causes any flow control window to exceed the maximum size as a connection error (Section 5.4.1) of type FLOW_CONTROL_ERROR.

    终端必须将SETTINGS_INITIAL_WINDOW_SIZE的修改导致流量控制窗口超过最大值的情况作为类型为流量控制错误的连接错误(章节 5.4.1)处理。

    .9.3 Reducing the Stream Window Size 减少流量窗口大小

    A receiver that wishes to use a smaller flow control window than the current size can send a new SETTINGS frame. However, the receiver MUST be prepared to receive data that exceeds this window size, since the sender might send data that exceeds the lower limit prior to processing the SETTINGS frame.


    After sending a SETTINGS frame that reduces the initial flow control window size, a receiver has two options for handling streams that exceed flow control limits:


    1. The receiver can immediately send RST_STREAM with FLOW_CONTROL_ERROR error code for the affected streams.
    2. The receiver can accept the streams and tolerate the resulting head of line blocking, sending WINDOW_UPDATE frames as it consumes data.

    3. 接收端可以针对受影响的流立即发送带有流量控制错误错误码的RST_STREAM帧。

    4. 接收端如果在消耗数据可以接受流并且忍受报头阻塞的结果,并发送WINDOW_UPDATE帧。

    .10 CONTINUATION 延续帧

    The CONTINUATION frame (type=0x9) is used to continue a sequence of header block fragments (Section 4.3). Any number of CONTINUATION frames can be sent on an existing stream, as long as the preceding frame is on the same stream and is a HEADERS, PUSH_PROMISE or CONTINUATION frame without the END_HEADERS flag set.


      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     |                   Header Block Fragment (*)                 ...

    The CONTINUATION frame payload contains a header block fragment (Section 4.3).


    The CONTINUATION frame defines the following flag:

    • END_HEADERS (0x4):Bit 3 being set indicates that this frame ends a header block (Section 4.3). If the END_HEADERS bit is not set, this frame MUST be followed by another CONTINUATION frame. A receiver MUST treat the receipt of any other type of frame or a frame on a different stream as a connection error (Section 5.4.1) of type PROTOCOL_ERROR.


    • END_HEADERS (0x4) : 位3设置指示这个帧的报头区块的终止(章节4.3)。 如果END_HEADERS位没有被设置,这个帧必须跟着另一个延续帧。接收到必须将收到其他类型的帧或者其他流上的帧错位类型为协议错误的连接错误(章节5.4.1)处理。

    The CONTINUATION frame changes the connection state as defined in Section 4.3.


    CONTINUATION frames MUST be associated with a stream. If a CONTINUATION frame is received whose stream identifier field is 0x0, the recipient MUST respond with a connection error (Section 5.4.1) of type PROTOCOL_ERROR.


    A CONTINUATION frame MUST be preceded by a HEADERS, PUSH_PROMISE or CONTINUATION frame without the END_HEADERS flag set. A recipient that observes violation of this rule MUST respond with a connection error (Section 5.4.1) of type PROTOCOL_ERROR.


     Error Codes 错误码

    Error codes are 32-bit fields that are used in RST_STREAM and GOAWAY frames to convey the reasons for the stream or connection error.


    Error codes share a common code space. Some error codes apply only to either streams or the entire connection and have no defined semantics in the other context.


    The following error codes are defined:

    • NO_ERROR (0x0):The associated condition is not as a result of an error. For example, a GOAWAY might include this code to indicate graceful shutdown of a connection.
    • PROTOCOL_ERROR (0x1):The endpoint detected an unspecific protocol error. This error is for use when a more specific error code is not available.
    • INTERNAL_ERROR (0x2):The endpoint encountered an unexpected internal error.
    • FLOW_CONTROL_ERROR (0x3):The endpoint detected that its peer violated the flow control protocol.
    • SETTINGS_TIMEOUT (0x4):The endpoint sent a SETTINGS frame, but did not receive a response in a timely manner. See Settings Synchronization (Section 6.5.3).
    • STREAM_CLOSED (0x5):The endpoint received a frame after a stream was half closed.
    • FRAME_SIZE_ERROR (0x6):The endpoint received a frame that was larger than the maximum size that it supports.
    • REFUSED_STREAM (0x7):The endpoint refuses the stream prior to performing any application processing, see Section 8.1.4 for details.
    • CANCEL (0x8):Used by the endpoint to indicate that the stream is no longer needed.
    • COMPRESSION_ERROR (0x9):The endpoint is unable to maintain the header compression context for the connection.
    • CONNECT_ERROR (0xa):The connection established in response to a CONNECT request (Section 8.3) was reset or abnormally closed.
    • ENHANCE_YOUR_CALM (0xb):The endpoint detected that its peer is exhibiting a behavior that might be generating excessive load.
    • INADEQUATE_SECURITY (0xc):The underlying transport has properties that do not meet minimum security requirements (see Section 9.2).


    • NO_ERROR (0) : 相关的条件并不是错误的结果。例如超时帧可以携带此错误码指示连接的平滑关闭。
    • PROTOCOL_ERROR (1) : 终端检测到一个不确定的协议错误。这个错误用在一个更具体的错误码不可用的时候。
    • INTERNAL_ERROR (2) : 终端遇到意外的内部错误。
    • FLOW_CONTROL_ERROR (3) : 终端检测到对等端违反了流量控制协议。
    • SETTINGS_TIMEOUT (4) : 终端发送了设置帧,但是没有及时收到响应。见Settings Synchronization。
    • STREAM_CLOSED (5) : 终端在流半封闭的时候收到帧。
    • FRAME_SIZE_ERROR (6) : 终端收到大小超过最大尺寸的帧。
    • REFUSED_STREAM (7) : 终端拒绝流在它执行任何应用处理之前,详见Reliability(章节 8.1.4)
    • CANCEL (8) : 终端使用这个标示某个流不再需要。
    • COMPRESSION_ERROR (9) : 终端无法维持报头压缩上下文的连接
    • CONNECT_ERROR (10) : 响应某个连接请求建立的连接被服为异常关闭。
    • ENHANCE_YOUR_CALM (11) : 终端检测出对等端在表现出可能会产生过大负荷的行为。
    • INADEQUATE_SECURITY (12) : 基础传输包含属性不满足文档或者终端申明的最小要求。

    Unknown or unsupported error codes MUST NOT trigger any special behavior. These MAY be treated by an implementation as being equivalent to INTERNAL_ERROR.


    8 HTTP Message Exchanges HTTP消息交换

    HTTP/2 is intended to be as compatible as possible with current uses of HTTP. This means that, from the application perspective, the features of the protocol are largely unchanged. To achieve this, all request and response semantics are preserved, although the syntax of conveying those semantics has changed.


    Thus, the specification and requirements of HTTP/1.1 Semantics and Content [RFC7231], Conditional Requests [RFC7232], Range Requests [RFC7233], Caching [RFC7234] and Authentication [RFC7235] are applicable to HTTP/2. Selected portions of HTTP/1.1 Message Syntax and Routing [RFC7230], such as the HTTP and HTTPS URI schemes, are also applicable in HTTP/2, but the expression of those semantics for this protocol are defined in the sections below.

    因此,HTTP/1.1语义与内容、有条件的请求、范围请求、缓存与验证定义的规范与要求同样适用于HTTP/2。HTTP/1.1消息语法与路由选定的内容,例如HTTP与HTTPS URI方案,也同样适用于HTTP/2,但是表达这些协议的语义在下面的章节定义。

    8.1 HTTP Request/Response Exchange HTTP 请求/响应交换

    A client sends an HTTP request on a new stream, using a previously unused stream identifier (Section 5.1.1). A server sends an HTTP response on the same stream as the request.


    An HTTP message (request or response) consists of:

    1. one HEADERS frame (followed by zero or more CONTINUATION frames) containing the message headers (see [RFC7230], Section 3.2), and
    2. zero or more DATA frames containing the message payload (see [RFC7230], Section 3.3), and
    3. optionally, one HEADERS frame, followed by zero or more CONTINUATION frames containing the trailer-part, if present (see [RFC7230], Section 4.1.2).


    1. 一个报头帧,后面跟着0个或多个延续帧(包含消息报头;见RFC7230),以及
    2. 0个或多个数据帧(包含消息载荷,见RFC7230章节3.3),以及
    3. 一个可选的版头,后面跟着0个或多个延续帧(如果存在,包含尾部部分,见RFC7230章节4.1.2)

    The last frame in the sequence bears an END_STREAM flag, noting that a HEADERS frame bearing the END_STREAM flag can be followed by CONTINUATION frames that carry any remaining portions of the header block.


    Other frames (from any stream) MUST NOT occur between either HEADERS frame and any CONTINUATION frames that might follow.


    Otherwise, frames MAY be interspersed on the stream between these frames, but those frames do not carry HTTP semantics. In particular, HEADERS frames (and any CONTINUATION frames that follow) other than the first and optional last frames in this sequence do not carry HTTP semantics.


    Trailing header fields are carried in a header block that also terminates the stream. That is, a sequence starting with a HEADERS frame, followed by zero or more CONTINUATION frames, where the HEADERS frame bears an END_STREAM flag. Header blocks after the first that do not terminate the stream are not part of an HTTP request or response.


    An HTTP request/response exchange fully consumes a single stream. A request starts with the HEADERS frame that puts the stream into an "open" state. The request ends with a frame bearing END_STREAM, which causes the stream to become "half closed (local)" for the client and "half closed (remote)" for the server. A response starts with a HEADERS frame and ends with a frame bearing END_STREAM, which places the stream in the "closed" state.

    一个HTTP 请求/响应的数据交换在同一个流上进行。一个请求由是流进入打开状态的报头帧开始,并由一个携带使流对客户端进入半封闭的END_STREAM标记的帧结束,另外可选的后面可以跟着延续帧,使流进入关闭状态。一个响应从HEADERS帧开始并且结束于一个放在封闭状态中支撑END_STREAM的帧

    8.1.1 Informational Responses 响应信息

    The 1xx series of HTTP response status codes ([RFC7231], Section 6.2) are not supported in HTTP/2.


    The most common use case for 1xx is using an Expect header field with a 100-continue token (colloquially, "Expect/continue") to indicate that the client expects a 100 (Continue) non-final response status code, receipt of which indicates that the client should continue sending the request body if it has not already done so.


    Typically, Expect/continue is used by clients wishing to avoid sending a large amount of data in a request body, only to have the request rejected by the origin server, thereby leaving the connection potentially unusable.


    HTTP/2 does not enable the Expect/continue mechanism; if the server sends a final status code to reject the request, it can do so without making the underlying connection unusable.


    Note that this means HTTP/2 clients sending requests with bodies may waste at least one round trip of sent data when the request is rejected. This can be mitigated by restricting the amount of data sent for the first round trip by bandwidth-constrained clients, in anticipation of a final status code.


    Other defined 1xx status codes are not applicable to HTTP/2. For example, the semantics of 101 (Switching Protocols) aren't suitable to a multiplexed protocol. Likewise, 102 (Processing) [RFC2518] is no longer necessary to ensure connection liveness, because HTTP/2 has a separate means of keeping the connection alive. The use of the 102 (Processing) status code for progress reporting has since been deprecated and is not retained.


    This difference between protocol versions necessitates special handling by intermediaries that translate between them:

    • An intermediary that translates HTTP/1.1 requests to HTTP/2 MUST generate a 100 (Continue) response if a received request includes and Expect header field with a 100-continue token ([RFC7231], Section 5.1.1), unless it can immediately generate a final status code. It MUST NOT forward the 100-continue expectation in the request header fields.
    • An intermediary that translates HTTP/2 to HTTP/1.1 MAY add an Expect header field with a 100-continue expectation when forwarding a request that has a body; see [RFC7231], Section 5.1.1 for specific requirements.
    • An intermediary that gateways HTTP/2 to HTTP/1.1 MUST discard all other 1xx informational responses.


    • 转换HTTP/1.1到HTTP/2的中介网关如果收到请求包含带有100-继续token(RFC7231,章节5.1.1)的期望报头字段,必须生成一个100(继续)响应,除非它能马上生成一个最终状态码。绝对不能转发请求报头中的100-继续期望字段。
    • 转换HTTP/2到HTTP/1.1的中介网关在转发一个带有正文的请求时可以添加一个带有100-继续的期望报头字段。特定要求见RFC7231.
    • 转换HTTP/2到HTTP/1.1的中介网关必须丢弃所有1xx以外的响应信息。

    8.1.2 HTTP Header Fields HTTP报头字段

    HTTP header fields carry information as a series of key-value pairs. For a listing of registered HTTP headers, see the Message Header Field Registry maintained at https://www.iana.org/assignments/message-headers.


    While HTTP/1.x used the message start-line (see [RFC7230], Section 3.1) to convey the target URI and method of the request, and the status code for the response, HTTP/2 uses special pseudo-headers beginning with ':' character (ASCII 0x3a) for this purpose.

    HTTP/1.x使用消息起始线(见RFC7230,章节3.1)来传达URI目标和请求的方法、以及响应的状态码,HTTP/2使用特殊的':'(ASCII 0x3a)开头的伪头字符来实现这个目的。

    Just as in HTTP/1.x, header field names are strings of ASCII characters that are compared in a case-insensitive fashion. However, header field names MUST be converted to lowercase prior to their encoding in HTTP/2. A request or response containing uppercase header field names MUST be treated as malformed (Section


    This means that an intermediary transforming an HTTP/1.x message to HTTP/2 will need to remove any header fields nominated by the Connection header field, along with the Connection header field itself. Such intermediaries SHOULD also remove other connection-specific header fields, such as Keep-Alive, Proxy-Connection, Transfer-Encoding and Upgrade, even if they are not nominated by Connection.


    One exception to this is the TE header field, which MAY be present in an HTTP/2 request, but when it is MUST NOT contain any value other than "trailers".

    一个例外是TE报头字段,这个可能在 HTTP/2 请求中保留,但是它不能包含“trailers”以外的值。

    Note: : HTTP/2 purposefully does not support upgrade to another protocol. The handshake methods described in Section 3 are believed sufficient to negotiate the use of alternative protocols.

    Note: : HTTP/2不支持升级到其他协议。3章节中描述的握手协议被认为足够用来作为替代协议使用。 Request Header Fields 请求报头字段

    HTTP/2 defines a number of pseudo header fields starting with a colon ':' character that carry information about the request target:

    • The :method header field includes the HTTP method ([RFC7231], Section 4).
    • The :scheme header field includes the scheme portion of the target URI ([RFC3986], Section 3.1). :scheme is not restricted to http and https schemed URIs. A proxy or gateway can translate requests for non-HTTP schemes, enabling the use of HTTP to interact with non-HTTP services.
    • The :authority header field includes the authority portion of the target URI ([RFC3986], Section 3.2). The authority MUST NOT include the deprecated userinfo subcomponent for http or https schemed URIs. To ensure that the HTTP/1.1 request line can be reproduced accurately, this header field MUST be omitted when translating from an HTTP/1.1 request that has a request target in origin or asterisk form (see [RFC7230], Section 5.3). Clients that generate HTTP/2 requests directly SHOULD instead omit the Host header field. An intermediary that converts an HTTP/2 request to HTTP/1.1 MUST create a Host header field if one is not present in a request by copying the value of the :authority header field.
    • The :path header field includes the path and query parts of the target URI (the path-absolute production from [RFC3986] and optionally a '?' character followed by the query production, see [RFC3986], Section 3.3 and [RFC3986], Section 3.4). This field MUST NOT be empty; URIs that do not contain a path component MUST include a value of '/', unless the request is an OPTIONS request in asterisk form, in which case the :path header field MUST include '*'.


    • :method 报头字段包含了HTTP方法
    • :scheme字段包含了目标URI方案部分。 :scheme并不是被限制于http和https类的URIS。代理或者网路关口可以转化非HTTP体系的请求使其可以于非HTTP得请求互动
    • :authority报头字段包含了目标URI的权限部分。这个权限绝对不能包含http:或者https: URIs的废弃的用户信息子成份。 为了保证HTTP/1.1请求行能被精确复制,当原始请求有请求目标或者星号形式(见[ http-p1 ],5.3节)的HTTP/1.1请求进行转换时这个字段必须被忽略。客户端直接生成HTTP/2请求的相反应该忽略Host报头字段。如果其中一个请求没有Host字段,中介端将HTTP/2请求转换为HTTP/1.1请求的时候必须复制:authority字段的值来生成Host字段。
    • :path字段包含目标URI的路径及查询部分(绝对路径由[RFC3986]以及可选的‘?’字符后面跟着查询词组成见xx)。这个字段绝对不能为空;URI不包含path组件的必须包含一个'/'值,除非请求是一个星号形式的可选请求:这种情况下:path报头字段必须包含""。是一个星号形式的可选请求:这种情况下:path报头字段必须包含""。 Response Header Fields 响应报头字段

    A single :status header field is defined that carries the HTTP status code field (see [RFC7231], Section 6). This header field MUST be included in all responses, otherwise the response is malformed (Section

    定义了一个单独的:status 报头字段携带了HTTP状态码信息(见RFC7231,章节6)。这个报头字段必须包含在所有的响应中,否则响应就是不规范的(章节8.1.2.5)。

    HTTP/2 does not define a way to carry the version or reason phrase that is included in an HTTP/1.1 status line.

    HTTP/2没有定义一种方式携带HTTP/1.1状态行中的版本或原因短语信息。 Header Field Ordering 报头字段顺序

    HTTP Header Compression [COMPRESSION] does not preserve the order of header fields, because the relative order of header fields with different names is not important. However, the same header field can be repeated to form a list (see [RFC7230], Section 3.2.2), where the relative order of header field values is significant. This repetition can occur either as a single header field with a comma-separated list of values, or as several header fields with a single value, or any combination thereof. Therefore, in the latter case, ordering needs to be preserved before compression takes place.


    To preserve the order of multiple occurrences of a header field with the same name, its ordered values are concatenated into a single value using a zero-valued octet (0x0) to delimit them.


    After decompression, header fields that have values containing zero octets (0x0) MUST be split into multiple header fields before being processed.


    For example, the following HTTP/1.x header block:


                  Content-Type: text/html
                  Cache-Control: max-age=60, private
                  Cache-Control: must-revalidate

    contains three Cache-Control directives; two directives in the first Cache-Control header field, and the third directive in the second Cache-Control field. Before compression, they would need to be converted to a form similar to this (with 0x0 represented as ''):


    cache-control: max-age=60, privatemust-revalidate
                  content-type: text/html

    Note here that the ordering between Content-Type and Cache-Control is not preserved, but the relative ordering of the Cache-Control directives - as well as the fact that the first two were comma-separated, while the last was on a different line - is.


    Header fields containing multiple values MUST be concatenated into a single value unless the ordering of that header field is known to be not significant.


    The special case of set-cookie - which does not form a comma-separated list, but can have multiple values - does not depend on ordering. The set-cookie header field MAY be encoded as multiple header field values, or as a single concatenated value.

    特殊情况是设置cookie——不需要形成一个逗号分隔的列表,但是可以有多个值——不需要依赖顺序。设置cookie字段可以被编码成多行报头字段值,或者单个的连接值。 Compressing the Cookie Header Field 报头Cookie字段压缩

    The Cookie header field [COOKIE] can carry a significant amount of redundant data.


    The Cookie header field uses a semi-colon (";") to delimit cookie-pairs (or "crumbs"). This header field doesn't follow the list construction rules in HTTP (see [RFC7230], Section 3.2.2), which prevents cookie-pairs from being separated into different name-value pairs. This can significantly reduce compression efficiency as individual cookie-pairs are updated.


    To allow for better compression efficiency, the Cookie header field MAY be split into separate header fields, each with one or more cookie-pairs. If there are multiple Cookie header fields after decompression, these MUST be concatenated into a single octet string using the two octet delimiter of 0x3B, 0x20 (the ASCII string "; ").

    为了更好的压缩效率,Cookie字段可以被分隔成多个报头字段,每个包含一个或者多个cookie对。如果解压后有多个Cookie报头字段,他们必须由两个字节的0x3B, 0x20(ASCII";")连接成单个字段。

    The Cookie header field MAY be split using a zero octet (0x0), as defined in Section When decoding, zero octets MUST be replaced with the cookie delimiter ("; ").


    Therefore, the following sets of Cookie header fields are semantically equivalent, though the final form might appear in a different order after compression and decompression.


      cookie: a=b; c=d; e=f
      cookie: a=bc=d; e=f
      cookie: a=b
      cookie: c=d
      cookie: e=f Malformed Messages 不规范的消息

    A malformed request or response is one that uses a valid sequence of HTTP/2 frames, but is otherwise invalid due to the presence of prohibited header fields, the absence of mandatory header fields, or the inclusion of uppercase header field names.


    A request or response that includes an entity body can include a content-length header field. A request or response is also malformed if the value of a content-length header field does not equal the sum of the DATA frame payload lengths that form the body.


    Intermediaries that process HTTP requests or responses (i.e., any intermediary not acting as a tunnel) MUST NOT forward a malformed request or response.


    Implementations that detect malformed requests or responses need to ensure that the stream ends. For malformed requests, a server MAY send an HTTP response prior to closing or resetting the stream. Clients MUST NOT accept a malformed response. Note that these requirements are intended to protect against several types of common attacks against HTTP; they are deliberately strict, because being permissive can expose implementations to these vulnerabilities.


    8.1.3 Examples 示例

    This section shows HTTP/1.1 requests and responses, with illustrations of equivalent HTTP/2 requests and responses.


    An HTTP GET request includes request header fields and no body and is therefore transmitted as a single HEADERS frame, followed by zero or more CONTINUATION frames containing the serialized block of request header fields. The HEADERS frame in the following has both the END_HEADERS and END_STREAM flags set; no CONTINUATION frames are sent:

    一个带有请求报头字段但没有正文的HTTP GET请求将被转换成一个单独的报头帧,后面跟着0个或者多个包含序列化的报头字段区块的延续帧。序列中最后一个报头帧将有END_HEADERS和END_STREAM标记。

      GET /resource HTTP/1.1           HEADERS
      Host: example.org          ==>     + END_STREAM
      Accept: image/jpeg                 + END_HEADERS
                                           :method = GET
                                           :scheme = https
                                           :path = /resource
                                           host = example.org
                                           accept = image/jpeg

    Similarly, a response that includes only response header fields is transmitted as a HEADERS frame (again, followed by zero or more CONTINUATION frames) containing the serialized block of response header fields.


      HTTP/1.1 304 Not Modified        HEADERS
      ETag: "xyzzy"              ==>     + END_STREAM
      Expires: Thu, 23 Jan ...           + END_HEADERS
                                           :status = 304
                                           etag: "xyzzy"
                                           expires: Thu, 23 Jan ...

    An HTTP POST request that includes request header fields and payload data is transmitted as one HEADERS frame, followed by zero or more CONTINUATION frames containing the request header fields, followed by one or more DATA frames, with the last CONTINUATION (or HEADERS) frame having the END_HEADERS flag set and the final DATA frame having the END_STREAM flag set:

    带有报头和载荷数据的POST HTTP请求将被转换成一个报头帧,后面跟着0个或者多个带有请求报头字段的延续帧,同时后面跟着一个或者多个数据帧。延续帧或者报头帧的最后一帧有END_HEADERS标记,最后一个数据帧拥有END_STREAM标记。

     POST /resource HTTP/1.1          HEADERS
      Host: example.org          ==>     - END_STREAM
      Content-Type: image/jpeg           + END_HEADERS
      Content-Length: 123                  :method = POST
                                           :scheme = https
      {binary data}                        :path = /resource
                                           :authority = example.org
                                           content-type = image/jpeg
                                           content-length = 123
                                         + END_STREAM
                                       {binary data}

    Note that data contributing to any given header field could be spread between header block fragments. The allocation of header fields to frames in this example is illustrative only.


    A response that includes header fields and payload data is transmitted as a HEADERS frame, followed by zero or more CONTINUATION frames, followed by one or more DATA frames, with the last DATA frame in the sequence having the END_STREAM flag set:


      HTTP/1.1 200 OK                  HEADERS
      Content-Type: image/jpeg   ==>     - END_STREAM
      Content-Length: 123                + END_HEADERS
                                           :status = 200
      {binary data}                        content-type = image/jpeg
                                           content-length = 123
                                         + END_STREAM
                                       {binary data}

    Trailing header fields are sent as a header block after both the request or response header block and all the DATA frames have been sent. The HEADERS frame starting the trailers header block has the END_STREAM flag set.


      HTTP/1.1 200 OK                  HEADERS
      Content-Type: image/jpeg   ==>     - END_STREAM
      Transfer-Encoding: chunked         + END_HEADERS
      Trailer: Foo                         :status        = 200
                                           content-length = 123
      123                                  content-type   = image/jpeg
      {binary data}                        trailer        = Foo
      Foo: bar                         DATA
                                         - END_STREAM
                                       {binary data}
                                         + END_STREAM
                                         + END_HEADERS
                                           foo: bar

    8.1.4 Request Reliability Mechanisms in HTTP/2 HTTP/2响应可靠性机制

    In HTTP/1.1, an HTTP client is unable to retry a non-idempotent request when an error occurs, because there is no means to determine the nature of the error. It is possible that some server processing occurred prior to the error, which could result in undesirable effects if the request were reattempted.


    HTTP/2 provides two mechanisms for providing a guarantee to a client that a request has not been processed:

    • The GOAWAY frame indicates the highest stream number that might have been processed. Requests on streams with higher numbers are therefore guaranteed to be safe to retry.
    • The REFUSED_STREAM error code can be included in a RST_STREAM frame to indicate that the stream is being closed prior to any processing having occurred. Any request that was sent on the reset stream can be safely retried.


    • 超时帧指示了流可能被处理的最大流流标示。在更大数字的流上的请求可以保证安全的重试。
    • RST_STREAM帧中可以包含REFUSED_STREAM错误码来指示流由于之前的处理正在关闭。重置流上的任何请求都可以安全重试。

    Requests that have not been processed have not failed; clients MAY automatically retry them, even those with non-idempotent methods.


    A server MUST NOT indicate that a stream has not been processed unless it can guarantee that fact. If frames that are on a stream are passed to the application layer for any stream, then REFUSED_STREAM MUST NOT be used for that stream, and a GOAWAY frame MUST include a stream identifier that is greater than or equal to the given stream identifier.


    In addition to these mechanisms, the PING frame provides a way for a client to easily test a connection. Connections that remain idle can become broken as some middleboxes (for instance, network address translators, or load balancers) silently discard connection bindings. The PING frame allows a client to safely test whether a connection is still active without sending a request.


    8.2 Server Push 服务端推送

    HTTP/2 enables a server to pre-emptively send (or "push") one or more associated responses to a client in response to a single request. This feature becomes particularly helpful when the server knows the client will need to have those responses available in order to fully process the response to the original request.


    Pushing additional responses is optional, and is negotiated between individual endpoints. The SETTINGS_ENABLE_PUSH setting can be set to 0 to indicate that server push is disabled.


    Because pushing responses is effectively hop-by-hop, an intermediary could receive pushed responses from the server and choose not to forward those on to the client. In other words, how to make use of the pushed responses is up to that intermediary. Equally, the intermediary might choose to push additional responses to the client, without any action taken by the server.


    A client cannot push. Thus, servers MUST treat the receipt of a PUSH_PROMISE frame as a connection error (Section 5.4.1) of type PROTOCOL_ERROR. Clients MUST reject any attempt to change the SETTINGS_ENABLE_PUSH setting to a value other than 0 by treating the message as a connection error (Section 5.4.1) of type PROTOCOL_ERROR.


    A server can only push responses that are cacheable (see [RFC7234], Section 3); promised requests MUST be safe (see [RFC7231], Section 4.2.1) and MUST NOT include a request body.


    8.2.1 Push Requests 推送请求

    Server push is semantically equivalent to a server responding to a request; however, in this case that request is also sent by the server, as a PUSH_PROMISE frame.


    The PUSH_PROMISE frame includes a header block that contains a complete set of request header fields that the server attributes to the request. It is not possible to push a response to a request that includes a request body.


    Pushed responses are always associated with an explicit request from the client. The PUSH_PROMISE frames sent by the server are sent on that explicit request's stream. The PUSH_PROMISE frame also includes a promised stream identifier, chosen from the stream identifiers available to the server (see Section 5.1.1).


    The header fields in PUSH_PROMISE and any subsequent CONTINUATION frames MUST be a valid and complete set of request header fields (Section The server MUST include a method in the :method header field that is safe and cacheable. If a client receives a PUSH_PROMISE that does not include a complete and valid set of header fields, or the :method header field identifies a method that is not safe, it MUST respond with a stream error (Section 5.4.2) of type PROTOCOL_ERROR.


    The server SHOULD send PUSH_PROMISE (Section 6.6) frames prior to sending any frames that reference the promised responses. This avoids a race where clients issue requests prior to receiving any PUSH_PROMISE frames.


    For example, if the server receives a request for a document containing embedded links to multiple image files, and the server chooses to push those additional images to the client, sending push promises before the DATA frames that contain the image links ensures that the client is able to see the promises before discovering embedded links. Similarly, if the server pushes responses referenced by the header block (for instance, in Link header fields), sending the push promises before sending the header block ensures that clients do not request them.

    例如:如果服务端收到文档请求包含多个嵌入式的图像链接,而且服务端选择推送那些额外的图像给客户端,再数据帧前发送push promises能确保客户端能够在发现内嵌链接前看到这些承诺。类似的,如果服务端推送与报头区块(例如,在Link报头域)相关的响应,再发送报头区块前推送承诺能确保客户端不请求它们。

    PUSH_PROMISE frames MUST NOT be sent by the client.


    PUSH_PROMISE frames can be sent by the server in response to any client-initiated stream, but the stream MUST be in either the "open" or "half closed (remote)" state with respect to the server. PUSH_PROMISE frames are interspersed with the frames that comprise a response, though they cannot be interspersed with HEADERS and CONTINUATION frames that comprise a single header block.


    Sending a PUSH_PROMISE frame creates a new stream and puts the stream into the “reserved (local)” state for the server and the “reserved (remote)” state for the client.


    8.2.2 Push Responses 推送响应

    After sending the PUSH_PROMISE frame, the server can begin delivering the pushed response as a response (Section on a server-initiated stream that uses the promised stream identifier. The server uses this stream to transmit an HTTP response, using the same sequence of frames as defined in Section 8.1. This stream becomes "half closed" to the client (Section 5.1) after the initial HEADERS frame is sent.


    Once a client receives a PUSH_PROMISE frame and chooses to accept the pushed response, the client SHOULD NOT issue any requests for the promised response until after the promised stream has closed.


    If the client determines, for any reason, that it does not wish to receive the pushed response from the server, or if the server takes too long to begin sending the promised response, the client can send an RST_STREAM frame, using either the CANCEL or REFUSED_STREAM codes, and referencing the pushed stream's identifier.


    A client can use the SETTINGS_MAX_CONCURRENT_STREAMS setting to limit the number of responses that can be concurrently pushed by a server. Advertising a SETTINGS_MAX_CONCURRENT_STREAMS value of zero disables server push by preventing the server from creating the necessary streams. This does not prohibit a server from sending PUSH_PROMISE frames; clients need to reset any promised streams that are not wanted.


    Clients receiving a pushed response MUST validate that the server is authorized to provide the response, see Section 10.1. For example, a server that offers a certificate for only the example.com DNS-ID or Common Name is not permitted to push a response for https://www.example.org/doc.


    The response for a PUSH_PROMISE stream begins with a HEADERS frame, which immediately puts the stream into the “half closed (remote)” state for the server and “half closed (local)” state for the client, and ends with a frame bearing END_STREAM, which places the stream in the "closed" state.


    Note:The client never sends a frame with the END_STREAM flag for a server push.


    8.3 The CONNECT Method CONNECT方法

    In HTTP/1.x, the pseudo-method CONNECT ([RFC7231], Section 4.3.6) is used to convert an HTTP connection into a tunnel to a remote host. CONNECT is primarily used with HTTP proxies to establish a TLS session with an origin server for the purposes of interacting with https resources.


    In HTTP/2, the CONNECT method is used to establish a tunnel over a single HTTP/2 stream to a remote host, for similar purposes. The HTTP header field mapping works as mostly as defined in Request Header Fields (Section, with a few differences. Specifically:


    • The :method header field is set to CONNECT.
    • The :scheme and :path header fields MUST be omitted.
    • The :authority header field contains the host and port to connect to (equivalent to the authority-form of the request-target of CONNECT requests, see [RFC7230], Section 5.3).

    • :method是连接中包含:method报头字段。

    • :scheme和:path报头字段必须被忽略。
    • :authority报头字段包含主机及连接的端口(相当于authority形式的请求目标连接请求,见RFC7230章节5.3)。

    A proxy that supports CONNECT establishes a TCP connection [TCP] to the server identified in the :authority header field. Once this connection is successfully established, the proxy sends a HEADERS frame containing a 2xx series status code to the client, as defined in [RFC7231], Section 4.3.6.


    After the initial HEADERS frame sent by each peer, all subsequent DATA frames correspond to data sent on the TCP connection. The payload of any DATA frames sent by the client are transmitted by the proxy to the TCP server; data received from the TCP server is assembled into DATA frames by the proxy. Frame types other than DATA or stream management frames (RST_STREAM, WINDOW_UPDATE, and PRIORITY) MUST NOT be sent on a connected stream, and MUST be treated as a stream error (Section 5.4.2) if received.


    The TCP connection can be closed by either peer. The END_STREAM flag on a DATA frame is treated as being equivalent to the TCP FIN bit. A client is expected to send a DATA frame with the END_STREAM flag set after receiving a frame bearing the END_STREAM flag. A proxy that receives a DATA frame with the END_STREAM flag set sends the attached data with the FIN bit set on the last TCP segment. A proxy that receives a TCP segment with the FIN bit set sends a DATA frame with the END_STREAM flag set. Note that the final TCP segment or DATA frame could be empty.

    TCP连接可以被各个对等端关闭。数据帧上的END_STREAM标记被认为与TCP FIN比特相同。客户端在收到带有END_STREAM标记的帧后被期望应该发送一个带有 END_STREAM标记的数据帧。代理接收到带有END_STREAM 标记的数据帧将在发送这些数据的时候在最后的TCP段上设置FIN位。带有接收到带有FIN位的TCP端发送一个带有END_STREAM标记的数据帧。注意最后的TCP端或者数据帧可以为空。

    A TCP connection error is signaled with RST_STREAM. A proxy treats any error in the TCP connection, which includes receiving a TCP segment with the RST bit set, as a stream error (Section 5.4.2) of type CONNECT_ERROR. Correspondingly, a proxy MUST send a TCP segment with the RST bit set if it detects an error with the stream or the HTTP/2 connection.


    9 Additional HTTP Requirements/Considerations 额外HTTP要求/考虑

    This section outlines attributes of the HTTP protocol that improve interoperability, reduce exposure to known security vulnerabilities, or reduce the potential for implementation variation.


    9.1 Connection Management 连接管理

    HTTP/2 connections are persistent. For best performance, it is expected clients will not close connections until it is determined that no further communication with a server is necessary (for example, when a user navigates away from a particular web page), or until the server closes the connection.


    Clients SHOULD NOT open more than one HTTP/2 connection to a given host and port pair, where host is derived from a URI, a selected alternative service [ALT-SVC], or a configured proxy.


    A client can create additional connections as replacements, either to replace connections that are near to exhausting the available stream identifier space (Section 5.1.1), to refresh the keying material for a TLS connection, or to replace connections that have encountered errors (Section 5.4.1).


    A client MAY open multiple connections to the same IP address and TCP port using different Server Name Indication [TLS-EXT] values or to provide different TLS client certificates, but SHOULD avoid creating multiple connections with the same configuration.


    Servers are encouraged to maintain open connections for as long as possible, but are permitted to terminate idle connections if necessary. When either endpoint chooses to close the transport-level TCP connection, the terminating endpoint SHOULD first send a GOAWAY (Section 6.8) frame so that both endpoints can reliably determine whether previously sent frames have been processed and gracefully complete or terminate any necessary remaining tasks.


    9.1.1 Connection Reuse 复用连接

    Clients MAY use a single server connection to send requests for URIs with multiple different authority components as long as the server is authoritative (Section 10.1). For http resources, this depends on the host having resolved to the same IP address.

    客户端可以使用单个服务端连接来发送多个不同认证组件的URIs请求,只要服务端是认证的(章节 10.1)。对于http资源来说,这个取决于对同个IP地址已经解析的主机端。

    For https resources, connection reuse additionally depends on having a certificate that is valid for the host in the URI. That is the use of server certificate with multiple subjectAltName attributes, or names with wildcards. For example, a certificate with a subjectAltName of *.example.com might permit the use of the same connection for a.example.com and b.example.com.


    In some deployments, reusing a connection for multiple origins can result in requests being directed to the wrong origin server. For example, TLS termination might be performed by a middlebox that uses the TLS Server Name Indication (SNI) [TLS-EXT] extension to select the an origin server. This means that it is possible for clients to send confidential information to servers that might not be the intended target for the request, even though the server has valid authentication credentials.


    A server that does not wish clients to reuse connections can indicate that it is not authoritative for a request by sending a 421 (Not Authoritative) status code in response to the request (see Section 9.1.2).


    9.1.2 The 421 (Not Authoritative) Status Code 421(未验证)状态码

    The 421 (Not Authoritative) status code indicates that the current origin server is not authoritative for the requested resource, in the sense of [RFC7230], Section 9.1 (see also Section 10.1).


    Clients receiving a 421 (Not Authoritative) response from a server MAY retry the request - whether the request method is idempotent or not - over a different connection. This is possible if a connection is reused (Section 9.1.1) or if an alternative service is selected ([ALT-SVC]).


    This status code MUST NOT be generated by proxies.


    A 421 response is cacheable by default; i.e., unless otherwise indicated by the method definition or explicit cache controls (see Section 4.2.2 of [RFC7234]).

    421响应缓存是默认的;举例:除非被定义的方法或显示的缓存控制另有说明(RFC7234, 章节4.2.2)。

    9.2 Use of TLS Features 使用TLS功能

    Implementations of HTTP/2 MUST support TLS 1.2 [TLS12] for HTTP/2 over TLS. The general TLS usage guidance in [TLSBCP] SHOULD be followed, with some additional restrictions that are specific to HTTP/2.

    实现HTTP/2必须支持TLS 1.2。通用的TLS用法指导应该遵循,同时加上对HTTP/2的特定支持。

    9.2.1 TLS Features TLS功能

    The TLS implementation MUST support the Server Name Indication (SNI) [TLS-EXT] extension to TLS. HTTP/2 clients MUST indicate the target domain name when negotiating TLS.


    The TLS implementation MUST disable compression. TLS compression can lead to the exposure of information that would not otherwise be revealed [RFC3749]. Generic compression is unnecessary since HTTP/2 provides compression features that are more aware of context and therefore likely to be more appropriate for use for performance, security or other reasons.


    The TLS implementation MUST disable renegotiation. An endpoint MUST treat a TLS renegotiation as a connection error (Section 5.4.1) of type PROTOCOL_ERROR. Note that disabling renegotiation can result in long-lived connections becoming unusable due to limits on the number of messages the underlying cipher suite can encipher.


    A client MAY use renegotiation to provide confidentiality protection for client credentials offered in the handshake, but any renegotiation MUST occur prior to sending the connection preface. A server SHOULD request a client certificate if it sees a renegotiation request immediately after establishing a connection.


    This effectively prevents the use of renegotiation in response to a request for a specific protected resource. A future specification might provide a way to support this use case.


    9.2.2 TLS Cipher Suites TLS加密套件

    The set of TLS cipher suites that are permitted in HTTP/2 is restricted. HTTP/2 MUST only be used with cipher suites that have ephemeral key exchange, such as the ephemeral Diffie-Hellman (DHE) [TLS12] or the elliptic curve variant (ECDHE) [RFC4492]. Ephemeral key exchange MUST have a minimum size of 2048 bits for DHE or security level of 128 bits for ECDHE. Clients MUST accept DHE sizes of up to 4096 bits. HTTP MUST NOT be used with cipher suites that use stream or block ciphers. Authenticated Encryption with Additional Data (AEAD) modes, such as the Galois Counter Model (GCM) mode for AES [RFC5288] are acceptable.

    HTTP/2中授权使用的TLS加密套件是不对外公开的。HTTP/2必须仅在支持密钥交换的加密套件下使用,如短暂的Diffie-Hellman(DHE)或椭圆曲线的变体(ecdhe)。交换的密钥必须具有最小尺寸的2048位(DHE)或128位的安全级别(ecdhe)。客户端必须接受多达4096位DHE尺寸。HTTP绝对不能使用用流或区块密码的加密套件。使用额外数据(AEAD)模式的认证加密,例如针对AES[ RFC5288 ] 的Galois Counter Model (GCM)模式是允许的。

    Clients MAY advertise support of other cipher suites in order to allow for connection to servers that do not support HTTP/2 to complete without the additional latency imposed by using a separate connection for fallback.


    An implementation SHOULD NOT negotiate a TLS connection for HTTP/2 without also negotiating a cipher suite that meets these requirements. Due to implementation limitations, it might not be possible to fail TLS negotiation. An endpoint MUST immediately terminate an HTTP/2 connection that does not meet these minimum requirements with a connection error (Section 5.4.1) of type INADEQUATE_SECURITY.


    9.3 GZip Content-Encoding 内部编码Gzip压缩


    10 Security Considerations 安全性考虑

    10.1 Server Authority 服务端认证

    A client is only able to accept HTTP/2 responses from servers that are authoritative for those resources. This is particularly important for server push (Section 8.2), where the client validates the PUSH_PROMISE before accepting the response.


    HTTP/2 relies on the HTTP/1.1 definition of authority for determining whether a server is authoritative in providing a given response, see [RFC7230], Section 9.1. This relies on local name resolution for the "http" URI scheme, and the authenticated server identity for the "https" scheme (see [RFC2818], Section 3).

    HTTP/2依据HTTP/1.1权限定义来检测服务端是否有权限提供给定的响应,见RFC2818 章节3.这依赖于本地“HTTP”URI方案的域名解析,以及服务端提供的“https”方案验证。

    A client MUST discard responses provided by a server that is not authoritative for those resources.


    10.2 Cross-Protocol Attacks 跨协议攻击

    In a cross-protocol attack, an attacker causes a client to initiate a transaction in one protocol toward a server that understands a different protocol. An attacker might be able to cause the transaction to appear as valid transaction in the second protocol. In combination with the capabilities of the web context, this can be used to interact with poorly protected servers in private networks.


    Completing a TLS handshake with an ALPN identifier for HTTP/2 can be considered sufficient protection against cross protocol attacks. ALPN provides a positive indication that a server is willing to proceed with HTTP/2, which prevents attacks on other TLS-based protocols.


    The encryption in TLS makes it difficult for attackers to control the data which could be used in a cross-protocol attack on a cleartext protocol.


    The cleartext version of HTTP/2 has minimal protection against cross-protocol attacks. The connection preface (Section 3.5) contains a string that is designed to confuse HTTP/1.1 servers, but no special protection is offered for other protocols. A server that is willing to ignore parts of an HTTP/1.1 request containing an Upgrade header field in addition to the client connection preface could be exposed to a cross-protocol attack.


    10.3 中介端封装攻击

    HTTP/2 header field names and values are encoded as sequences of octets with a length prefix. This enables HTTP/2 to carry any string of octets as the name or value of a header field. An intermediary that translates HTTP/2 requests or responses into HTTP/1.1 directly could permit the creation of corrupted HTTP/1.1 messages. An attacker might exploit this behavior to cause the intermediary to create HTTP/1.1 messages with illegal header fields, extra header fields, or even new messages that are entirely falsified.


    Header field names or values that contain characters not permitted by HTTP/1.1, including carriage return (ASCII 0xd) or line feed (ASCII 0xa) MUST NOT be translated verbatim by an intermediary, as stipulated in [RFC7230], Section 3.2.4.


    Translation from HTTP/1.x to HTTP/2 does not produce the same opportunity to an attacker. Intermediaries that perform translation to HTTP/2 MUST remove any instances of the obs-fold production from header field values.


    10.4 推送响应的缓存

    Pushed responses do not have an explicit request from the client; the request is provided by the server in the PUSH_PROMISE frame.


    Caching responses that are pushed is possible based on the guidance provided by the origin server in the Cache-Control header field. However, this can cause issues if a single server hosts more than one tenant. For example, a server might offer multiple users each a small portion of its URI space.


    Where multiple tenants share space on the same server, that server MUST ensure that tenants are not able to push representations of resources that they do not have authority over. Failure to enforce this would allow a tenant to provide a representation that would be served out of cache, overriding the actual representation that the authoritative tenant provides.


    Pushed responses for which an origin server is not authoritative (see Section 10.1) are never cached or used.


    10.5 拒绝服务的注意事项

    An HTTP/2 connection can demand a greater commitment of resources to operate than a HTTP/1.1 connection. The use of header compression and flow control depend on a commitment of resources for storing a greater amount of state. Settings for these features ensure that memory commitments for these features are strictly bounded.


    The number of PUSH_PROMISE frames is not constrained in the same fashion. A client that accepts server push SHOULD limit the number of streams it allows to be in the "reserved (remote)" state. Excessive number of server push streams can be treated as a stream error (Section 5.4.2) of type ENHANCE_YOUR_CALM.


    Processing capacity cannot be guarded as effectively as state capacity.


    The SETTINGS frame can be abused to cause a peer to expend additional processing time. This might be done by pointlessly changing SETTINGS parameters, setting multiple undefined parameters, or changing the same setting multiple times in the same frame. WINDOW_UPDATE or PRIORITY frames can be abused to cause an unnecessary waste of resources.

    设置帧可能被滥用导致对等端花费额外的处理时间。这可能是毫无意义改变设置参数、设置多个未定义的参数,或者在同个帧中多次修改同个值。WINDOW_UPDATE 或 PRIORITY帧也可能被滥用导致资源的不必要的浪费。服务端可能在没有权限为客户端产生过量工作时错误地假定源服务器的ALTSVC帧。

    Large numbers of small or empty frames can be abused to cause a peer to expend time processing frame headers. Note however that some uses are entirely legitimate, such as the sending of an empty DATA frame to end a stream.


    Header compression also offers some opportunities to waste processing resources; see Section 8 of [COMPRESSION] for more details on potential abuses.


    Limits in SETTINGS parameters cannot be reduced instantaneously, which leaves an endpoint exposed to behavior from a peer that could exceed the new limits. In particular, immediately after establishing a connection, limits set by a server are not known to clients and could be exceeded without being an obvious protocol violation.


    All these features - i.e., SETTINGS changes, small frames, header compression - have legitimate uses. These features become a burden only when they are used unnecessarily or to excess.


    An endpoint that doesn't monitor this behavior exposes itself to a risk of denial of service attack. Implementations SHOULD track the use of these features and set limits on their use. An endpoint MAY treat activity that is suspicious as a connection error (Section 5.4.1) of type ENHANCE_YOUR_CALM.


    10.5.1 Limits on Header Block Size

    A large header block (Section 4.3) can cause an implementation to commit a large amount of state. In servers and intermediaries, header fields that are critical to routing, such as :authority, :path, and :scheme are not guaranteed to be present early in the header block. In particular, values that are in the reference set cannot be emitted until the header block ends.

    一个较大的报头区块(章节4.3)可以导致具体实现中提交大量的状态。在服务端和中介端,报头字段对路由至关重要,例如:authority, :path, 和:scheme并没有保证在报头区块中靠前呈现。特别是,引用集合中的值只有在报头区块结束才能被提交。

    This can prevent streaming of the header fields to their ultimate destination, and forces the endpoint to buffer the entire header block. Since there is no hard limit to the size of a header block, an endpoint could be forced to exhaust available memory.


    A server that receives a larger header block than it is willing to handle can send an HTTP 431 (Request Header Fields Too Large) status code [RFC6585]. A client can discard responses that it cannot process. The header block MUST be processed to ensure a consistent connection state, unless the connection is closed.

    服务端接收到大于它愿意处理的报头区块可以发送一个HTTP 431(请求报头字段过大)状态码 RFC6585。终端可以丢弃其不能处理的响应。报头区块必须被处理以确保连续的连接状态,除非连接是关闭的。

    10.6 Use of Compression

    HTTP/2 enables greater use of compression for both header fields (Section 4.3) and entity bodies. Compression can allow an attacker to recover secret data when it is compressed in the same context as data under attacker control.


    There are demonstrable attacks on compression that exploit the characteristics of the web (e.g., [BREACH]). The attacker induces multiple requests containing varying plaintext, observing the length of the resulting ciphertext in each, which reveals a shorter length when a guess about the secret is correct.


    Implementations communicating on a secure channel MUST NOT compress content that includes both confidential and attacker-controlled data unless separate compression dictionaries are used for each source of data. Compression MUST NOT be used if the source of data cannot be reliably determined.


    Further considerations regarding the compression of header fields are described in [COMPRESSION].


    10.7 填充的使用

    Padding within HTTP/2 is not intended as a replacement for general purpose padding, such as might be provided by TLS [TLS12]. Redundant padding could even be counterproductive. Correct application can depend on having specific knowledge of the data that is being padded.

    HTTP/2的填充不打算作为通用填充的替换,如可能在TLS [TLS12]提供的。多余的填充可能会适得其反。正确的应用依靠的是对于填充的数据有具体的认知。

    To mitigate attacks that rely on compression, disabling or limiting compression might be preferable to padding as a countermeasure.


    Padding can be used to obscure the exact size of frame content, and is provided to mitigate specific attacks within HTTP. For example, attacks where compressed content includes both attacker-controlled plaintext and secret data (see for example, [BREACH]).


    Use of padding can result in less protection than might seem immediately obvious. At best, padding only makes it more difficult for an attacker to infer length information by increasing the number of frames an attacker has to observe. Incorrectly implemented padding schemes can be easily defeated. In particular, randomized padding with a predictable distribution provides very little protection; similarly, padding payloads to a fixed size exposes information as payload sizes cross the fixed size boundary, which could be possible if an attacker can control plaintext.


    Intermediaries SHOULD retain padding for DATA frames, but MAY drop padding for HEADERS and PUSH_PROMISE frames. A valid reason for an intermediary to change the amount of padding of frames is to improve the protections that padding provides.


    10.8 隐私注意事项

    Several characteristics of HTTP/2 provide an observer an opportunity to correlate actions of a single client or server over time. This includes the value of settings, the manner in which flow control windows are managed, the way priorities are allocated to streams, timing of reactions to stimulus, and handling of any optional features.


    As far as this creates observable differences in behavior, they could be used as a basis for fingerprinting a specific client, as defined in Section 1.8 of [HTML5].

    至于这个可观察到的行为的差异,他们可以作为特定的客户端的识别的基础,见HTML5 章节1.8中定义。

    11 IANA Considerations

    A string for identifying HTTP/2 is entered into the "Application Layer Protocol Negotiation (ALPN) Protocol IDs" registry established in [TLSALPN].

    标识HTTP/2的字符串已经收录到TLSALPN中建立的"Application Layer Protocol Negotiation (ALPN) Protocol IDs"注册表中。

    This document establishes a registry for frame types, settings, and error codes. These new registries are entered into a new "Hypertext Transfer Protocol (HTTP) 2 Parameters" section.

    本文档建立了帧类型、设置和错误码的记录。这些新的记录被收录到新的"Hypertext Transfer Protocol (HTTP) 2 Parameters" 章节中。

    This document registers the HTTP2-Settings header field for use in HTTP; and the 421 (Not Authoritative) status code.


    This document registers the PRI method for use in HTTP, to avoid collisions with the connection preface (Section 3.5).

    为了避免与连接序言(章节3.5)冲突, 本文档注册了在HTTP中使用的PRI方法。

    11.1 HTTP/2识别字符串的注册

    This document creates two registrations for the identification of HTTP/2 in the "Application Layer Protocol Negotiation (ALPN) Protocol IDs" registry established in [TLSALPN].


    The "h2" string identifies HTTP/2 when used over TLS:

    Protocol:HTTP/2 over TLS Identification Sequence:0x68 0x32 ("h2") Specification:This document The "h2c" string identifies HTTP/2 when used over cleartext TCP:Protocol:HTTP/2 over TCP Identification Sequence:0x68 0x32 0x63 ("h2c") Specification:This document


    • 协议: TLS上的HTTP/2
    • 标识序列 : 0x68 0x32 ("h2")
    • 定义: 本文档
    • "h2c"字符串标识使用明文TCP时的HTTP/2: TCP上的HTTP/2
    • 协议: TCP上的HTTP/2
    • 标识序列: 0x68 0x32 0x63 ("h2c")
    • 定义: 本文档

    11.2 Frame Type Registry

    This document establishes a registry for HTTP/2 frame types codes. The "HTTP/2 Frame Type" registry manages an 8-bit space. The "HTTP/2 Frame Type" registry operates under either of the "IETF Review" or "IESG Approval" policies [RFC5226] for values between 0x00 and 0xef, with values between 0xf0 and 0xff being reserved for experimental use.

    本文档建立了HTTP/2帧类型码的注册表。"HTTP/2 Frame Type"注册表管理一个8位的空间。" “HTTP/2 Frame Type” 注册表在 “IETF Review” 或者 “IESG Approval”政策下操作0x00 到 0xef的值, 0xf0 到 0xff的值是预留给实验用的。

    New entries in this registry require the following information:

    Frame Type:A name or label for the frame type. Code:The 8-bit code assigned to the frame type. Specification:A reference to a specification that includes a description of the frame layout, it's semantics and flags that the frame type uses, including any parts of the frame that are conditionally present based on the value of flags.


    • 帧类型:帧类型的名称或者标记
    • 编码:帧类型的8位编码标识
    • 定义:包含帧使用的布局、语义及标记的描述的定义参考规范,包含任何帧根据标记值有条件呈现的部分。

    The entries in the following table are registered by this document.


    Frame Type Code Section
    DATA 0x0 Section 6.1
    HEADERS 0x1 Section 6.2
    PRIORITY 0x2 Section 6.3
    RST_STREAM 0x3 Section 6.4
    SETTINGS 0x4 Section 6.5
    PUSH_PROMISE 0x5 Section 6.6
    PING 0x6 Section 6.7
    GOAWAY 0x7 Section 6.8
    WINDOW_UPDATE 0x8 Section 6.9
    CONTINUATION 0x9 Section 6.10

    11.3 Settings Registry

    This document establishes a registry for HTTP/2 settings. The "HTTP/2 Settings" registry manages a 16-bit space. The "HTTP/2 Settings" registry operates under the "Expert Review" policy [RFC5226] for values in the range from 0x0000 to 0xefff, with values between and 0xf000 and 0xffff being reserved for experimental use.

    本文档为HTTP/2设置建立了注册表。"HTTP/2 Settings"注册表管理了一个16位的空间。"HTTP/2 Settings"注册表在"Expert Review"政策(RFC5226)下操作0x0000 到 0xefff的值,0xf000 到 0xffff的值是为实验保留的。

    New registrations are advised to provide the following information:

    • Name:A symbolic name for the setting. Specifying a setting name is optional.
    • Code:The 16-bit code assigned to the setting.
    • Initial Value:An initial value for the setting.
    • Specification:A stable reference to a specification that describes the use of the setting.


    • 名称 : 设置的语义名称。定义一个设置的名称是可选的
    • 代码: 16位的指定设置的编码
    • 初始值: 设置的初始值
    • 说明:描述设置使用的稳定的参考定义

    An initial set of setting registrations can be found in Section 6.5.2.


    Name Code Initial Value Specification
    HEADER_TABLE_SIZE 0x1 4096 Section 6.5.2
    ENABLE_PUSH 0x2 1 Section 6.5.2
    MAX_CONCURRENT_STREAMS 0x3 (infinite) Section 6.5.2
    INITIAL_WINDOW_SIZE 0x4 65535 Section 6.5.2

    11.4 错误码注册

    This document establishes a registry for HTTP/2 error codes. The "HTTP/2 Error Code" registry manages a 32-bit space. The "HTTP/2 Error Code" registry operates under the "Expert Review" policy [RFC5226].


    Registrations for error codes are required to include a description of the error code. An expert reviewer is advised to examine new registrations for possible duplication with existing error codes. Use of existing registrations is to be encouraged, but not mandated.


    New registrations are advised to provide the following information:

    • Name:A name for the error code. Specifying an error code name is optional.
    • Code:The 32-bit error code value.
    • Description:A brief description of the error code semantics, longer if no detailed specification is provided.
    • Specification:An optional reference for a specification that defines the error code.

    The entries in the following table are registered by this document.


    • 错误码:32位错误码值
    • 名称: 错误码名称。指定一个错误码的名称是可选的。
    • 描述: 描述错误码的适用条件
    • 规范: 定义错误码规范的可选参考。


    Name Code Description Specification
    NO_ERROR 0x0 Graceful shutdown Section 7
    PROTOCOL_ERROR 0x1 Protocol error detected Section 7
    INTERNAL_ERROR 0x2 Implementation fault Section 7
    FLOW_CONTROL_ERROR 0x3 Flow control limits exceeded Section 7
    SETTINGS_TIMEOUT 0x4 Settings not acknowledged Section 7
    STREAM_CLOSED 0x5 Frame received for closed stream Section 7
    FRAME_SIZE_ERROR 0x6 Frame size incorrect Section 7
    REFUSED_STREAM 0x7 Stream not processed Section 7
    CANCEL 0x8 Stream cancelled Section 7
    COMPRESSION_ERROR 0x9 Compression state not updated Section 7
    CONNECT_ERROR 0xa TCP connection error for CONNECT method Section 7
    ENHANCE_YOUR_CALM 0xb Processing capacity exceeded Section 7
    INADEQUATE_SECURITY 0xc Negotiated TLS parameters not acceptable Section 7

    11.5 HTTP2设置报头字段注册

    This section registers the HTTP2-Settings header field in the Permanent Message Header Field Registry [BCP90].

    • Header field name:HTTP2-Settings
    • Applicable protocol:http
    • Status:standard
    • Author/Change controller:IETF
    • Specification document(s):Section 3.2.1 of this document
    • Related information:This header field is only used by an HTTP/2 client for Upgrade-based negotiation.


    • 报头字段名称: HTTP2-Settings
    • 应用层协议: http
    • 状态: 标准
    • 作者/修改操作者:LETF
    • 文档定义: 本文档3.2.1章节
    • 相关信息: 本字段只在HTTP/2客户端基于升级的协商中使用。

    11.6 PRI方法注册

    This section registers the PRI method in the HTTP Method Registry ([RFC7231], Section 8.1).

    Method Name:PRI Safe : No Idempotent : No Specification document(s) : Section 3.5 of this document Related information: This method is never used by an actual client. This method will appear to be used when an HTTP/1.1 server or intermediary attempts to parse an HTTP/2 connection preface.


    • 方法名称 : PRI
    • - 安全: 否
    • 幂等元 : 否
    • 文档定义: 本文档3.5章节
    • 相关信息: 这个方法从不会被确切的客户端使用。该方法只在HTTP/1.1服务端或者中介端试图解析HTTP/2连接序言中使用。

    11.7 The 421 Not Authoritative HTTP Status Code

    This document registers the 421 (Not Authoritative) HTTP Status code in the Hypertext Transfer Protocol (HTTP) Status Code Registry ([RFC7231], Section 8.2).

    • Status Code:421
    • Short Description: Not Authoritative
    • Specification: Section 9.1.2 of this document

    本文档在Hypertext Transfer Protocol (HTTP)状态码注册表(RFC7231,见章节8.2)中注册了HTTP 421(未验证)状态码。

    • 状态码:421
    • 简短描述: 未验证权限
    • 定义: 本文档章节9.1.2

    12. Acknowledgements

    This document includes substantial input from the following individuals:


    • Adam Langley, Wan-Teh Chang, Jim Morrison, Mark Nottingham, Alyssa Wilk, Costin Manolache, William Chan, Vitaliy Lvin, Joe Chan, Adam Barth, Ryan Hamilton, Gavin Peters, Kent Alstad, Kevin Lindsay, Paul Amer, Fan Yang, Jonathan Leighton (SPDY contributors).
    • Gabriel Montenegro and Willy Tarreau (Upgrade mechanism).
    • William Chan, Salvatore Loreto, Osama Mazahir, Gabriel Montenegro, Jitu Padhye, Roberto Peon, Rob Trace (Flow control).
    • Mike Bishop (Extensibility).
    • Mark Nottingham, Julian Reschke, James Snell, Jeff Pinner, Mike Bishop, Herve Ruellan (Substantial editorial contributions).
    • Alexey Melnikov was an editor of this document during 2013.
    • A substantial proportion of Martin's contribution was supported by Microsoft during his employment there.
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